React Js Interview Questions and Answers

Learning tech skills are excellent, and something you should do immediately! But once you learn them, it’s time to start implementing and examining for tech jobs. I have scoured the internet to pull collectively complete lists of popular React JS interview questions which are asked during React JS developer job interviews. And also tips on how you should plan and prepare for and answer them.

Table of Contents

These are scoured from React Interview Questions and Answers

Core React

1. What is React?

   React is an open-source frontend JavaScript library which is used for building user interfaces especially for single page applications. It is used for handling view layer for web and mobile apps. React was created by Jordan Walke, a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.

[↑] Back to top

2. What are the major features of React?

   The major features of React are:

   • It uses VirtualDOM instead of RealDOM considering that RealDOM manipulations are expensive.
   • Supports server-side rendering.
   • Follows Unidirectional data flow or data binding.
   • Uses reusable/composable UI components to develop the view.

[↑] Back to top

3. What is JSX?

   JSX is a XML-like syntax extension to ECMAScript (the acronym stands for JavaScript XML). Basically it just provides syntactic sugar for the React.createElement() function, giving us expressiveness of JavaScript along with HTML like template syntax.

   In the example below text inside <h1> tag is returned as JavaScript function to the render function.

   class App extends React.Component {
     render() {
       return (
         <div>
           <h1>{"Welcome to React world!"}</h1>
         </div>
       );
     }
   }

[↑] Back to top

4. What is the difference between Element and Component?

   An Element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other Elements in their props. Creating a React element is cheap. Once an element is created, it is never mutated.

   The object representation of React Element would be as follows:

   const element = React.createElement("div", { id: "login-btn" }, "Login");

   The above React.createElement() function returns an object:

   {
     type: 'div',
     props: {
       children: 'Login',
       id: 'login-btn'
     }
   }

   And finally it renders to the DOM using ReactDOM.render():

   <div id="login-btn">Login</div>

   Whereas a component can be declared in several different ways. It can be a class with a render() method. Alternatively, in simple cases, it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:

   const Button = ({ onLogin }) => (
     <div id={"login-btn"} onClick={onLogin}>
       Login
     </div>
   );

   Then JSX gets transpiled to a React.createElement() function tree:

   const Button = ({ onLogin }) =>
     React.createElement("div", { id: "login-btn", onClick: onLogin }, "Login");

[↑] Back to top

5. How to create components in React?

   There are two possible ways to create a component.

   1. Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as first parameter and return React elements:

      function Greeting({ message }) {
        return <h1>{`Hello, ${message}`}</h1>;
      }

   2. Class Components: You can also use ES6 class to define a component. The above function component can be written as:

      class Greeting extends React.Component {
        render() {
          return <h1>{`Hello, ${this.props.message}`}</h1>;
        }
      }

[↑] Back to top

6. When to use a Class Component over a Function Component?

   If the component needs state or lifecycle methods then use class component otherwise use function component. However, from React 16.8 with the addition of Hooks, you could use state , lifecycle methods and other features that were only available in class component right in your function component.

[↑] Back to top

7. What are Pure Components?

   React.PureComponent is exactly the same as React.Component except that it handles the shouldComponentUpdate() method for you. When props or state changes, PureComponent will do a shallow comparison on both props and state. Component on the other hand won't compare current props and state to next out of the box. Thus, the component will re-render by default whenever shouldComponentUpdate is called.

[↑] Back to top

8. What is state in React?

   State of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components.

   Let's create an user component with message state,

   class User extends React.Component {
     constructor(props) {
       super(props)
   
       this.state = {
         message: 'Welcome to React world'
       }
     }
   
     render() {
       return (
         <div>
           <h1>{this.state.message}</h1>
         </div>
       )
     }
   }

   

   State is similar to props, but it is private and fully controlled by the component. i.e, It is not accessible to any component other than the one that owns and sets it.

[↑] Back to top

9. What are props in React?

   Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.

   The primary purpose of props in React is to provide following component functionality:

   1. Pass custom data to your component.
   2. Trigger state changes.
   3. Use via this.props.reactProp inside component's render() method.

   For example, let us create an element with reactProp property:

   <Element reactProp={"1"} />

   This reactProp (or whatever you came up with) name then becomes a property attached to React's native props object which originally already exists on all components created using React library.

   props.reactProp

[↑] Back to top

10. What is the difference between state and props?

    Both props and state are plain JavaScript objects. While both of them hold information that influences the output of render, they are different in their functionality with respect to component. Props get passed to the component similar to function parameters whereas state is managed within the component similar to variables declared within a function.

[↑] Back to top

11. Why should we not update the state directly?

    If you try to update state directly then it won't re-render the component.

    //Wrong
    this.state.message = "Hello world";

    Instead use setState() method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.

    //Correct
    this.setState({ message: "Hello World" });

    Note: You can directly assign to the state object either in constructor or using latest javascript's class field declaration syntax.

[↑] Back to top

12. What is the purpose of callback function as an argument of setState()?

    The callback function is invoked when setState finished and the component gets rendered. Since setState() is asynchronous the callback function is used for any post action.

    Note: It is recommended to use lifecycle method rather than this callback function.

    setState({ name: "John" }, () =>
      console.log("The name has updated and component re-rendered")
    );

[↑] Back to top

13. What is the difference between HTML and React event handling?

    Below are some of the main differences between HTML and React event handling,

    1. In HTML, the event name should be in lowercase:

       <button onclick="activateLasers()"></button>

       Whereas in React it follows camelCase convention:

       <button onClick={activateLasers}>

    2. In HTML, you can return false to prevent default behavior:

       <a
         href="#"
         onclick='console.log("The link was clicked."); return false;'
       />

       Whereas in React you must call preventDefault() explicitly:

       function handleClick(event) {
         event.preventDefault();
         console.log("The link was clicked.");
       }

    3. In HTML, you need to invoke the function by appending () Whereas in react you should not append () with the function name. (refer "activateLasers" function in the first point for example)

[↑] Back to top

14. How to bind methods or event handlers in JSX callbacks?

    There are 3 possible ways to achieve this:

    1. Binding in Constructor: In JavaScript classes, the methods are not bound by default. The same thing applies for React event handlers defined as class methods. Normally we bind them in constructor.

       class Component extends React.Componenet {
         constructor(props) {
           super(props);
           this.handleClick = this.handleClick.bind(this);
         }
       
         handleClick() {
           // ...
         }
       }

    2. Public class fields syntax: If you don't like to use bind approach then public class fields syntax can be used to correctly bind callbacks.

       handleClick = () => {
         console.log("this is:", this);
       };

       <button onClick={this.handleClick}>{"Click me"}</button>

    3. Arrow functions in callbacks: You can use arrow functions directly in the callbacks.

       <button onClick={(event) => this.handleClick(event)}>
         {"Click me"}
       </button>

    Note: If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with .bind() or public class fields syntax approach considering performance.

[↑] Back to top

15. How to pass a parameter to an event handler or callback?

    You can use an arrow function to wrap around an event handler and pass parameters:

    <button onClick={() => this.handleClick(id)} />

    This is an equivalent to calling .bind:

    <button onClick={this.handleClick.bind(this, id)} />

    Apart from these two approaches, you can also pass arguments to a function which is defined as array function

    <button onClick={this.handleClick(id)} />;
    handleClick = (id) => () => {
      console.log("Hello, your ticket number is", id);
    };

[↑] Back to top

16. What are synthetic events in React?

    SyntheticEvent is a cross-browser wrapper around the browser's native event. It's API is same as the browser's native event, including stopPropagation() and preventDefault(), except the events work identically across all browsers.

[↑] Back to top

17. What are inline conditional expressions?

    You can use either if statements or ternary expressions which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator &&.

    <h1>Hello!</h1>;
    {
      messages.length > 0 && !isLogin ? (
        <h2>You have {messages.length} unread messages.</h2>
      ) : (
        <h2>You don't have unread messages.</h2>
      );
    }

[↑] Back to top

18. What is "key" prop and what is the benefit of using it in arrays of elements?

    A key is a special string attribute you should include when creating arrays of elements. Key prop helps React identify which items have changed, are added, or are removed.

    Most often we use ID from our data as key:

    const todoItems = todos.map((todo) => <li key={todo.id}>{todo.text}</li>);

    When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:

    const todoItems = todos.map((todo, index) => (
      <li key={index}>{todo.text}</li>
    ));

    Note:

    1. Using indexes for keys is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
    2. If you extract list item as separate component then apply keys on list component instead of li tag.
    3. There will be a warning message in the console if the key prop is not present on list items.

[↑] Back to top

19. What is the use of refs?

    The ref is used to return a reference to the element. They should be avoided in most cases, however, they can be useful when you need a direct access to the DOM element or an instance of a component.

[↑] Back to top

20. How to create refs?

    There are two approaches

    1. This is a recently added approach. Refs are created using React.createRef() method and attached to React elements via the ref attribute. In order to use refs throughout the component, just assign the ref to the instance property within constructor.

       class MyComponent extends React.Component {
         constructor(props) {
           super(props);
           this.myRef = React.createRef();
         }
         render() {
           return <div ref={this.myRef} />;
         }
       }

    2. You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element accessed as follows,

       class SearchBar extends Component {
         constructor(props) {
           super(props);
           this.txtSearch = null;
           this.state = { term: "" };
           this.setInputSearchRef = (e) => {
             this.txtSearch = e;
           };
         }
         onInputChange(event) {
           this.setState({ term: this.txtSearch.value });
         }
         render() {
           return (
             <input
               value={this.state.term}
               onChange={this.onInputChange.bind(this)}
               ref={this.setInputSearchRef}
             />
           );
         }
       }

    You can also use refs in function components using closures. Note: You can also use inline ref callbacks even though it is not a recommended approach

[↑] Back to top

21. What are forward refs?

    Ref forwarding is a feature that lets some components take a ref they receive, and pass it further down to a child.

    const ButtonElement = React.forwardRef((props, ref) => (
      <button ref={ref} className="CustomButton">
        {props.children}
      </button>
    ));
    
    // Create ref to the DOM button:
    const ref = React.createRef();
    <ButtonElement ref={ref}>{"Forward Ref"}</ButtonElement>;

[↑] Back to top

22. Which is preferred option with in callback refs and findDOMNode()?

    It is preferred to use callback refs over findDOMNode() API. Because findDOMNode() prevents certain improvements in React in the future.

    The legacy approach of using findDOMNode:

    class MyComponent extends Component {
      componentDidMount() {
        findDOMNode(this).scrollIntoView();
      }
    
      render() {
        return <div />;
      }
    }

    The recommended approach is:

    class MyComponent extends Component {
      constructor(props) {
        super(props);
        this.node = createRef();
      }
      componentDidMount() {
        this.node.current.scrollIntoView();
      }
    
      render() {
        return <div ref={this.node} />;
      }
    }

[↑] Back to top

23. Why are String Refs legacy?

    If you worked with React before, you might be familiar with an older API where the ref attribute is a string, like ref={'textInput'}, and the DOM node is accessed as this.refs.textInput. We advise against it because string refs have below issues, and are considered legacy. String refs were removed in React v16.

    1. They force React to keep track of currently executing component. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.

    2. They are not composable — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.

    3. They don't work with static analysis like Flow. Flow can't guess the magic that framework does to make the string ref appear on this.refs, as well as its type (which could be different). Callback refs are friendlier to static analysis.

    4. It doesn't work as most people would expect with the "render callback" pattern (e.g. )

       class MyComponent extends Component {
         renderRow = (index) => {
           // This won't work. Ref will get attached to DataTable rather than MyComponent:
           return <input ref={"input-" + index} />;
       
           // This would work though! Callback refs are awesome.
           return <input ref={(input) => (this["input-" + index] = input)} />;
         };
       
         render() {
           return (
             <DataTable data={this.props.data} renderRow={this.renderRow} />
           );
         }
       }

[↑] Back to top

24. What is Virtual DOM?

    The Virtual DOM (VDOM) is an in-memory representation of Real DOM. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called reconciliation.

[↑] Back to top

25. How Virtual DOM works?

    The Virtual DOM works in three simple steps.

    1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.

       

    2. Then the difference between the previous DOM representation and the new one is calculated.

       

    3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed.

       

[↑] Back to top

26. What is the difference between Shadow DOM and Virtual DOM?

    The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.

[↑] Back to top

27. What is React Fiber?

    Fiber is the new reconciliation engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.

[↑] Back to top

28. What is the main goal of React Fiber?

    The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.

[↑] Back to top

29. What are controlled components?

    A component that controls the input elements within the forms on subsequent user input is called Controlled Component, i.e, every state mutation will have an associated handler function.

    For example, to write all the names in uppercase letters, we use handleChange as below,

    handleChange(event) {
      this.setState({value: event.target.value.toUpperCase()})
    }

[↑] Back to top

30. What are uncontrolled components?

    The Uncontrolled Components are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.

    In the below UserProfile component, the name input is accessed using ref.

    class UserProfile extends React.Component {
      constructor(props) {
        super(props);
        this.handleSubmit = this.handleSubmit.bind(this);
        this.input = React.createRef();
      }
    
      handleSubmit(event) {
        alert("A name was submitted: " + this.input.current.value);
        event.preventDefault();
      }
    
      render() {
        return (
          <form onSubmit={this.handleSubmit}>
            <label>
              {"Name:"}
              <input type="text" ref={this.input} />
            </label>
            <input type="submit" value="Submit" />
          </form>
        );
      }
    }

    In most cases, it's recommend to use controlled components to implement forms.

[↑] Back to top

31. What is the difference between createElement and cloneElement?

    JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the object representation of UI. Whereas cloneElement is used to clone an element and pass it new props.

[↑] Back to top

32. What is Lifting State Up in React?

    When several components need to share the same changing data then it is recommended to lift the shared state up to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.

[↑] Back to top

33. What are the different phases of component lifecycle?

    The component lifecycle has three distinct lifecycle phases:

    1. Mounting: The component is ready to mount in the browser DOM. This phase covers initialization from constructor(), getDerivedStateFromProps(), render(), and componentDidMount() lifecycle methods.

    2. Updating: In this phase, the component get updated in two ways, sending the new props and updating the state either from setState() or forceUpdate(). This phase covers getDerivedStateFromProps(), shouldComponentUpdate(), render(), getSnapshotBeforeUpdate() and componentDidUpdate() lifecycle methods.

    3. Unmounting: In this last phase, the component is not needed and get unmounted from the browser DOM. This phase includes componentWillUnmount() lifecycle method.

    It's worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows

    1. Render The component will render without any side-effects. This applies for Pure components and in this phase, React can pause, abort, or restart the render.

    2. Pre-commit Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the getSnapshotBeforeUpdate().

    3. Commit React works with the DOM and executes the final lifecycles respectively componentDidMount() for mounting, componentDidUpdate() for updating, and componentWillUnmount() for unmounting.

    React 16.3+ Phases (or an interactive version)

    

    Before React 16.3

    

[↑] Back to top

34. What are the lifecycle methods of React?

    Before React 16.3

    • componentWillMount: Executed before rendering and is used for App level configuration in your root component.
    • componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
    • componentWillReceiveProps: Executed when particular prop updates to trigger state transitions.
    • shouldComponentUpdate: Determines if the component will be updated or not. By default it returns true. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
    • componentWillUpdate: Executed before re-rendering the component when there are props & state changes confirmed by shouldComponentUpdate() which returns true.
    • componentDidUpdate: Mostly it is used to update the DOM in response to prop or state changes.
    • componentWillUnmount: It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.

    React 16.3+

    • getDerivedStateFromProps: Invoked right before calling render() and is invoked on every render. This exists for rare use cases where you need derived state. Worth reading if you need derived state.
    • componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
    • shouldComponentUpdate: Determines if the component will be updated or not. By default it returns true. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
    • getSnapshotBeforeUpdate: Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into componentDidUpdate(). This is useful to capture information from the DOM i.e. scroll position.
    • componentDidUpdate: Mostly it is used to update the DOM in response to prop or state changes. This will not fire if shouldComponentUpdate() returns false.
    • componentWillUnmount It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.

[↑] Back to top

35. What are Higher-Order Components?

    A higher-order component (HOC) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.

    We call them pure components because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.

    const EnhancedComponent = higherOrderComponent(WrappedComponent);

    HOC can be used for many use cases:

    1. Code reuse, logic and bootstrap abstraction.
    2. Render hijacking.
    3. State abstraction and manipulation.
    4. Props manipulation.

[↑] Back to top

36. How to create props proxy for HOC component?

    You can add/edit props passed to the component using props proxy pattern like this:

    function HOC(WrappedComponent) {
      return class Test extends Component {
        render() {
          const newProps = {
            title: "New Header",
            footer: false,
            showFeatureX: false,
            showFeatureY: true,
          };
    
          return <WrappedComponent {...this.props} {...newProps} />;
        }
      };
    }

[↑] Back to top

37. What is context?

    Context provides a way to pass data through the component tree without having to pass props down manually at every level.

    For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.

    const { Provider, Consumer } = React.createContext(defaultValue);

[↑] Back to top

38. What is children prop?

    Children is a prop (this.prop.children) that allow you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component as children prop.

    There are a number of methods available in the React API to work with this prop. These include React.Children.map, React.Children.forEach, React.Children.count, React.Children.only, React.Children.toArray.

    A simple usage of children prop looks as below,

    const MyDiv = React.createClass({
      render: function () {
        return <div>{this.props.children}</div>;
      },
    });
    
    ReactDOM.render(
      <MyDiv>
        <span>{"Hello"}</span>
        <span>{"World"}</span>
      </MyDiv>,
      node
    );

[↑] Back to top

39. How to write comments in React?

    The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.

    Single-line comments:

    <div>
      {/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
      {`Welcome ${user}, let's play React`}
    </div>

    Multi-line comments:

    <div>
      {/* Multi-line comments for more than
       one line */}
      {`Welcome ${user}, let's play React`}
    </div>

[↑] Back to top

40. What is the purpose of using super constructor with props argument?

    A child class constructor cannot make use of this reference until super() method has been called. The same applies for ES6 sub-classes as well. The main reason of passing props parameter to super() call is to access this.props in your child constructors.

    Passing props:

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);
    
        console.log(this.props); // prints { name: 'John', age: 42 }
      }
    }

    Not passing props:

    class MyComponent extends React.Component {
      constructor(props) {
        super();
    
        console.log(this.props); // prints undefined
    
        // but props parameter is still available
        console.log(props); // prints { name: 'John', age: 42 }
      }
    
      render() {
        // no difference outside constructor
        console.log(this.props); // prints { name: 'John', age: 42 }
      }
    }

    The above code snippets reveals that this.props is different only within the constructor. It would be the same outside the constructor.

[↑] Back to top

41. What is reconciliation?

    When a component's props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. When they are not equal, React will update the DOM. This process is called reconciliation.

[↑] Back to top

42. How to set state with a dynamic key name?

    If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with computed property names.

    handleInputChange(event) {
      this.setState({ [event.target.id]: event.target.value })
    }

[↑] Back to top

43. What would be the common mistake of function being called every time the component renders?

    You need to make sure that function is not being called while passing the function as a parameter.

    render() {
      // Wrong: handleClick is called instead of passed as a reference!
      return <button onClick={this.handleClick()}>{'Click Me'}</button>
    }

    Instead, pass the function itself without parenthesis:

    render() {
      // Correct: handleClick is passed as a reference!
      return <button onClick={this.handleClick}>{'Click Me'}</button>
    }

[↑] Back to top

44. Is lazy function supports named exports?

    No, currently React.lazy function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components. Let's take a component file which exports multiple named components,

    // MoreComponents.js
    export const SomeComponent = /* ... */;
    export const UnusedComponent = /* ... */;

    and reexport MoreComponents.js components in an intermediate file IntermediateComponent.js

    // IntermediateComponent.js
    export { SomeComponent as default } from "./MoreComponents.js";

    Now you can import the module using lazy function as below,

    import React, { lazy } from "react";
    const SomeComponent = lazy(() => import("./IntermediateComponent.js"));

[↑] Back to top

45. Why React uses className over class attribute?

    class is a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principal reason why React uses className instead of class. Pass a string as the className prop.

    render() {
      return <span className={'menu navigation-menu'}>{'Menu'}</span>
    }

[↑] Back to top

46. What are fragments?

    It's common pattern in React which is used for a component to return multiple elements. Fragments let you group a list of children without adding extra nodes to the DOM.

    render() {
      return (
        <React.Fragment>
          <ChildA />
          <ChildB />
          <ChildC />
        </React.Fragment>
      )
    }

    There is also a shorter syntax, but it's not supported in many tools:

    render() {
      return (
        <>
          <ChildA />
          <ChildB />
          <ChildC />
        </>
      )
    }

[↑] Back to top

47. Why fragments are better than container divs?

    Below are the list of reasons,

    1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
    2. Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
    3. The DOM Inspector is less cluttered.

[↑] Back to top

48. What are portals in React?

    Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.

    ReactDOM.createPortal(child, container);

    The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.

[↑] Back to top

49. What are stateless components?

    If the behaviour is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.

[↑] Back to top

50. What are stateful components?

    If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are always class components and have a state that gets initialized in the constructor.

    class App extends Component {
      constructor(props) {
        super(props);
        this.state = { count: 0 };
      }
    
      render() {
        // ...
      }
    }

    React 16.8 Update:

    Hooks let you use state and other React features without writing classes.

    The Equivalent Functional Component

     import React, {useState} from 'react';
    
     const App = (props) => {
       const [count, setCount] = useState(0);
    
       return (
         // JSX
       )
     }

[↑] Back to top

51. How to apply validation on props in React?

    When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It's disabled in production mode due to performance impact. The mandatory props are defined with isRequired.

    The set of predefined prop types:

    1. PropTypes.number
    2. PropTypes.string
    3. PropTypes.array
    4. PropTypes.object
    5. PropTypes.func
    6. PropTypes.node
    7. PropTypes.element
    8. PropTypes.bool
    9. PropTypes.symbol
    10. PropTypes.any

    We can define propTypes for User component as below:

    import React from "react";
    import PropTypes from "prop-types";
    
    class User extends React.Component {
      static propTypes = {
        name: PropTypes.string.isRequired,
        age: PropTypes.number.isRequired,
      };
    
      render() {
        return (
          <>
            <h1>{`Welcome, ${this.props.name}`}</h1>
            <h2>{`Age, ${this.props.age}`}</h2>
          </>
        );
      }
    }

    Note: In React v15.5 PropTypes were moved from React.PropTypes to prop-types library.

[↑] Back to top

52. What are the advantages of React?

    Below are the list of main advantages of React,

    1. Increases the application's performance with Virtual DOM.
    2. JSX makes code easy to read and write.
    3. It renders both on client and server side (SSR).
    4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
    5. Easy to write unit and integration tests with tools such as Jest.

[↑] Back to top

53. What are the limitations of React?

    Apart from the advantages, there are few limitations of React too,

    1. React is just a view library, not a full framework.
    2. There is a learning curve for beginners who are new to web development.
    3. Integrating React into a traditional MVC framework requires some additional configuration.
    4. The code complexity increases with inline templating and JSX.
    5. Too many smaller components leading to over engineering or boilerplate.

[↑] Back to top

54. What are error boundaries in React v16?

    Error boundaries are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.

    A class component becomes an error boundary if it defines a new lifecycle method called componentDidCatch(error, info) or static getDerivedStateFromError():

    class ErrorBoundary extends React.Component {
      constructor(props) {
        super(props);
        this.state = { hasError: false };
      }
    
      componentDidCatch(error, info) {
        // You can also log the error to an error reporting service
        logErrorToMyService(error, info);
      }
    
      static getDerivedStateFromError(error) {
        // Update state so the next render will show the fallback UI.
        return { hasError: true };
      }
    
      render() {
        if (this.state.hasError) {
          // You can render any custom fallback UI
          return <h1>{"Something went wrong."}</h1>;
        }
        return this.props.children;
      }
    }

    After that use it as a regular component:

    <ErrorBoundary>
      <MyWidget />
    </ErrorBoundary>

[↑] Back to top

55. How error boundaries handled in React v15?

    React v15 provided very basic support for error boundaries using unstable_handleError method. It has been renamed to componentDidCatch in React v16.

[↑] Back to top

56. What are the recommended ways for static type checking?

    Normally we use PropTypes library (React.PropTypes moved to a prop-types package since React v15.5) for type checking in the React applications. For large code bases, it is recommended to use static type checkers such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.

[↑] Back to top

57. What is the use of react-dom package?

    The react-dom package provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:

    1. render()
    2. hydrate()
    3. unmountComponentAtNode()
    4. findDOMNode()
    5. createPortal()

[↑] Back to top

58. What is the purpose of render method of react-dom?

    This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.

    ReactDOM.render(element, container[, callback])

    If the optional callback is provided, it will be executed after the component is rendered or updated.

[↑] Back to top

59. What is ReactDOMServer?

    The ReactDOMServer object enables you to render components to static markup (typically used on node server). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:

    1. renderToString()
    2. renderToStaticMarkup()

    For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call renderToString to render your root component to a string, which you then send as response.

    // using Express
    import { renderToString } from "react-dom/server";
    import MyPage from "./MyPage";
    
    app.get("/", (req, res) => {
      res.write(
        "<!DOCTYPE html><html><head><title>My Page</title></head><body>"
      );
      res.write('<div id="content">');
      res.write(renderToString(<MyPage />));
      res.write("</div></body></html>");
      res.end();
    });

[↑] Back to top

60. How to use innerHTML in React?

    The dangerouslySetInnerHTML attribute is React's replacement for using innerHTML in the browser DOM. Just like innerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html object as key and HTML text as value.

    In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:

    function createMarkup() {
      return { __html: "First · Second" };
    }
    
    function MyComponent() {
      return <div dangerouslySetInnerHTML={createMarkup()} />;
    }

[↑] Back to top

61. How to use styles in React?

    The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.

    const divStyle = {
      color: "blue",
      backgroundImage: "url(" + imgUrl + ")",
    };
    
    function HelloWorldComponent() {
      return <div style={divStyle}>Hello World!</div>;
    }

    Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).

[↑] Back to top

62. How events are different in React?

    Handling events in React elements has some syntactic differences:

    1. React event handlers are named using camelCase, rather than lowercase.
    2. With JSX you pass a function as the event handler, rather than a string.

[↑] Back to top

63. What will happen if you use setState() in constructor?

    When you use setState(), then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: Can only update a mounted or mounting component. So we need to use this.state to initialize variables inside constructor.

[↑] Back to top

64. What is the impact of indexes as keys?

    Keys should be stable, predictable, and unique so that React can keep track of elements.

    In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.

    {
      todos.map((todo, index) => <Todo {...todo} key={index} />);
    }

    If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.

    {
      todos.map((todo) => <Todo {...todo} key={todo.id} />);
    }

[↑] Back to top

65. Is it good to use setState() in componentWillMount() method?

    It is recommended to avoid async initialization in componentWillMount() lifecycle method. componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in componentDidMount() instead of componentWillMount().

    componentDidMount() {
      axios.get(`api/todos`)
        .then((result) => {
          this.setState({
            messages: [...result.data]
          })
        })
    }

[↑] Back to top

66. What will happen if you use props in initial state?

    If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.

    The below component won't display the updated input value:

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);
    
        this.state = {
          records: [],
          inputValue: this.props.inputValue,
        };
      }
    
      render() {
        return <div>{this.state.inputValue}</div>;
      }
    }

    Using props inside render method will update the value:

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);
    
        this.state = {
          record: [],
        };
      }
    
      render() {
        return <div>{this.props.inputValue}</div>;
      }
    }

[↑] Back to top

67. How do you conditionally render components?

    In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.

    const MyComponent = ({ name, address }) => (
      <div>
        <h2>{name}</h2>
        {address && <p>{address}</p>}
      </div>
    );

    If you need an if-else condition then use ternary operator.

    const MyComponent = ({ name, address }) => (
      <div>
        <h2>{name}</h2>
        {address ? <p>{address}</p> : <p>{"Address is not available"}</p>}
      </div>
    );

[↑] Back to top

68. Why we need to be careful when spreading props on DOM elements?

    When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.

    For example,

    const ComponentA = () => (
      <ComponentB isDisplay={true} className={"componentStyle"} />
    );
    
    const ComponentB = ({ isDisplay, ...domProps }) => (
      <div {...domProps}>{"ComponentB"}</div>
    );

[↑] Back to top

69. How you use decorators in React?

    You can decorate your class components, which is the same as passing the component into a function. Decorators are flexible and readable way of modifying component functionality.

    @setTitle("Profile")
    class Profile extends React.Component {
      //....
    }
    
    /*
      title is a string that will be set as a document title
      WrappedComponent is what our decorator will receive when
      put directly above a component class as seen in the example above
    */
    const setTitle = (title) => (WrappedComponent) => {
      return class extends React.Component {
        componentDidMount() {
          document.title = title;
        }
    
        render() {
          return <WrappedComponent {...this.props} />;
        }
      };
    };

    Note: Decorators are a feature that didn't make it into ES7, but are currently a stage 2 proposal.

[↑] Back to top

70. How do you memoize a component?

    There are memoize libraries available which can be used on function components.

    For example moize library can memoize the component in another component.

    import moize from "moize";
    import Component from "./components/Component"; // this module exports a non-memoized component
    
    const MemoizedFoo = moize.react(Component);
    
    const Consumer = () => {
      <div>
        {"I will memoize the following entry:"}
        <MemoizedFoo />
      </div>;
    };

    Update: Since React v16.6.0, we have a React.memo. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.

    const MemoComponent = React.memo(function MemoComponent(props) {
      /* render using props */
    });
    OR;
    export default React.memo(MyFunctionComponent);

[↑] Back to top

71. How you implement Server Side Rendering or SSR?

    React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.

    import ReactDOMServer from "react-dom/server";
    import App from "./App";
    
    ReactDOMServer.renderToString(<App />);

    This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.

[↑] Back to top

72. How to enable production mode in React?

    You should use Webpack's DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.

[↑] Back to top

73. What is CRA and its benefits?

    The create-react-app CLI tool allows you to quickly create & run React applications with no configuration step.

    Let's create Todo App using CRA:

    # Installation
    $ npm install -g create-react-app
    
    # Create new project
    $ create-react-app todo-app
    $ cd todo-app
    
    # Build, test and run
    $ npm run build
    $ npm run test
    $ npm start

    It includes everything we need to build a React app:

    1. React, JSX, ES6, and Flow syntax support.
    2. Language extras beyond ES6 like the object spread operator.
    3. Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
    4. A fast interactive unit test runner with built-in support for coverage reporting.
    5. A live development server that warns about common mistakes.
    6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps.

[↑] Back to top

74. What is the lifecycle methods order in mounting?

    The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.

    1. constructor()
    2. static getDerivedStateFromProps()
    3. render()
    4. componentDidMount()

[↑] Back to top

75. What are the lifecycle methods going to be deprecated in React v16?

    The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.

    1. componentWillMount()
    2. componentWillReceiveProps()
    3. componentWillUpdate()

    Starting with React v16.3 these methods are aliased with UNSAFE_ prefix, and the unprefixed version will be removed in React v17.

[↑] Back to top

76. What is the purpose of getDerivedStateFromProps() lifecycle method?

    The new static getDerivedStateFromProps() lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or null to indicate that the new props do not require any state updates.

    class MyComponent extends React.Component {
      static getDerivedStateFromProps(props, state) {
        // ...
      }
    }

    This lifecycle method along with componentDidUpdate() covers all the use cases of componentWillReceiveProps().

[↑] Back to top

77. What is the purpose of getSnapshotBeforeUpdate() lifecycle method?

    The new getSnapshotBeforeUpdate() lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to componentDidUpdate().

    class MyComponent extends React.Component {
      getSnapshotBeforeUpdate(prevProps, prevState) {
        // ...
      }
    }

    This lifecycle method along with componentDidUpdate() covers all the use cases of componentWillUpdate().

[↑] Back to top

78. Do Hooks replace render props and higher order components?

    Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.

[↑] Back to top

79. What is the recommended way for naming components?

    It is recommended to name the component by reference instead of using displayName.

    Using displayName for naming component:

    export default React.createClass({
      displayName: "TodoApp",
      // ...
    });

    The recommended approach:

    export default class TodoApp extends React.Component {
      // ...
    }

[↑] Back to top

80. What is the recommended ordering of methods in component class?

    Recommended ordering of methods from mounting to render stage:

    1. static methods
    2. constructor()
    3. getChildContext()
    4. componentWillMount()
    5. componentDidMount()
    6. componentWillReceiveProps()
    7. shouldComponentUpdate()
    8. componentWillUpdate()
    9. componentDidUpdate()
    10. componentWillUnmount()
    11. click handlers or event handlers like onClickSubmit() or onChangeDescription()
    12. getter methods for render like getSelectReason() or getFooterContent()
    13. optional render methods like renderNavigation() or renderProfilePicture()
    14. render()

[↑] Back to top

81. What is a switching component?

    A switching component is a component that renders one of many components. We need to use object to map prop values to components.

    For example, a switching component to display different pages based on page prop:

    import HomePage from "./HomePage";
    import AboutPage from "./AboutPage";
    import ServicesPage from "./ServicesPage";
    import ContactPage from "./ContactPage";
    
    const PAGES = {
      home: HomePage,
      about: AboutPage,
      services: ServicesPage,
      contact: ContactPage,
    };
    
    const Page = (props) => {
      const Handler = PAGES[props.page] || ContactPage;
    
      return <Handler {...props} />;
    };
    
    // The keys of the PAGES object can be used in the prop types to catch dev-time errors.
    Page.propTypes = {
      page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
    };

[↑] Back to top

82. Why we need to pass a function to setState()?

    The reason behind for this is that setState() is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately after setState() is called. That means you should not rely on the current state when calling setState() since you can't be sure what that state will be. The solution is to pass a function to setState(), with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature of setState().

    Let's say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.

    // assuming this.state.count === 0
    this.setState({ count: this.state.count + 1 });
    this.setState({ count: this.state.count + 1 });
    this.setState({ count: this.state.count + 1 });
    // this.state.count === 1, not 3

    If we pass a function to setState(), the count gets incremented correctly.

    this.setState((prevState, props) => ({
      count: prevState.count + props.increment,
    }));
    // this.state.count === 3 as expected

    (OR)

    Why function is preferred over object for setState()?

    React may batch multiple setState() calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

    This counter example will fail to update as expected:

    // Wrong
    this.setState({
      counter: this.state.counter + this.props.increment,
    });

    The preferred approach is to call setState() with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.

    // Correct
    this.setState((prevState, props) => ({
      counter: prevState.counter + props.increment,
    }));

[↑] Back to top

[↑] Back to top

83. What is strict mode in React?

    React.StrictMode is a useful component for highlighting potential problems in an application. Just like <Fragment>, <StrictMode> does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for development mode only.

    import React from "react";
    
    function ExampleApplication() {
      return (
        <div>
          <Header />
          <React.StrictMode>
            <div>
              <ComponentOne />
              <ComponentTwo />
            </div>
          </React.StrictMode>
          <Footer />
        </div>
      );
    }

    In the example above, the strict mode checks apply to <ComponentOne> and <ComponentTwo> components only.

[↑] Back to top

84. What are React Mixins?

    Mixins are a way to totally separate components to have a common functionality. Mixins should not be used and can be replaced with higher-order components or decorators.

    One of the most commonly used mixins is PureRenderMixin. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:

    const PureRenderMixin = require("react-addons-pure-render-mixin");
    
    const Button = React.createClass({
      mixins: [PureRenderMixin],
      // ...
    });

[↑] Back to top

85. Why is isMounted() an anti-pattern and what is the proper solution?

    The primary use case for isMounted() is to avoid calling setState() after a component has been unmounted, because it will emit a warning.

    if (this.isMounted()) {
      this.setState({...})
    }

    Checking isMounted() before calling setState() does eliminate the warning, but it also defeats the purpose of the warning. Using isMounted() is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.

    An optimal solution would be to find places where setState() might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled in componentWillUnmount(), prior to unmounting.

[↑] Back to top

86. What are the Pointer Events supported in React?

    Pointer Events provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.

    The following event types are now available in React DOM:

    1. onPointerDown
    2. onPointerMove
    3. onPointerUp
    4. onPointerCancel
    5. onGotPointerCapture
    6. onLostPointerCapture
    7. onPointerEnter
    8. onPointerLeave
    9. onPointerOver
    10. onPointerOut

[↑] Back to top

87. Why should component names start with capital letter?

    If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.

    class SomeComponent extends Component {
      // Code goes here
    }

    You can define component class which name starts with lowercase letter, but when it's imported it should have capital letter. Here lowercase is fine:

    class myComponent extends Component {
      render() {
        return <div />;
      }
    }
    
    export default myComponent;

    While when imported in another file it should start with capital letter:

    import MyComponent from "./MyComponent";

[↑] Back to top

88. What are the exceptions on React component naming?

    The component names should start with a uppercase letter but there are few exceptions on this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.

    For example the below tag can be compiled to a valid component,

    render(){
       return (
           <obj.component /> // `React.createElement(obj.component)`
          )
    }

[↑] Back to top

88. Are custom DOM attributes supported in React v16?

    Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.

    For example, let's take a look at the below attribute:

    <div mycustomattribute={"something"} />

    Would render an empty div to the DOM with React v15:

    <div />

    In React v16 any unknown attributes will end up in the DOM:

    <div mycustomattribute="something" />

    This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.

[↑] Back to top

89. What is the difference between constructor and getInitialState?

    You should initialize state in the constructor when using ES6 classes, and getInitialState() method when using React.createClass().

    Using ES6 classes:

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);
        this.state = {
          /* initial state */
        };
      }
    }

    Using React.createClass():

    const MyComponent = React.createClass({
      getInitialState() {
        return {
          /* initial state */
        };
      },
    });

    Note: React.createClass() is deprecated and removed in React v16. Use plain JavaScript classes instead.

[↑] Back to top

90. Can you force a component to re-render without calling setState?

    By default, when your component's state or props change, your component will re-render. If your render() method depends on some other data, you can tell React that the component needs re-rendering by calling forceUpdate().

    component.forceUpdate(callback);

    It is recommended to avoid all uses of forceUpdate() and only read from this.props and this.state in render().

[↑] Back to top

91. What is the difference between super() and super(props) in React using ES6 classes?

    When you want to access this.props in constructor() then you should pass props to super() method.

    Using super(props):

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);
        console.log(this.props); // { name: 'John', ... }
      }
    }

    Using super():

    class MyComponent extends React.Component {
      constructor(props) {
        super();
        console.log(this.props); // undefined
      }
    }

    Outside constructor() both will display same value for this.props.

[↑] Back to top

92. How to loop inside JSX?

    You can simply use Array.prototype.map with ES6 arrow function syntax.

    For example, the items array of objects is mapped into an array of components:

    <tbody>
      {items.map((item) => (
        <SomeComponent key={item.id} name={item.name} />
      ))}
    </tbody>

    But you can't iterate using for loop:

    <tbody>
      for (let i = 0; i < items.length; i++) {
        <SomeComponent key={items[i].id} name={items[i].name} />
      }
    </tbody>

    This is because JSX tags are transpiled into function calls, and you can't use statements inside expressions. This may change thanks to do expressions which are stage 1 proposal.

[↑] Back to top

93. How do you access props in attribute quotes?

    React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:

    <img className="image" src="images/{this.props.image}" />

    But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:

    <img className="image" src={"images/" + this.props.image} />

    Using template strings will also work:

    <img className="image" src={`images/${this.props.image}`} />

[↑] Back to top

94. What is React proptype array with shape?

    If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape() as an argument to React.PropTypes.arrayOf().

    ReactComponent.propTypes = {
      arrayWithShape: React.PropTypes.arrayOf(
        React.PropTypes.shape({
          color: React.PropTypes.string.isRequired,
          fontSize: React.PropTypes.number.isRequired,
        })
      ).isRequired,
    };

[↑] Back to top

95. How to conditionally apply class attributes?

    You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.

    <div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">

    Instead you need to move curly braces outside (don't forget to include spaces between class names):

    <div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>

    Template strings will also work:

    <div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>

[↑] Back to top

96. What is the difference between React and ReactDOM?

    The react package contains React.createElement(), React.Component, React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The react-dom package contains ReactDOM.render(), and in react-dom/server we have server-side rendering support with ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().

[↑] Back to top

97. Why ReactDOM is separated from React?

    The React team worked on extracting all DOM-related features into a separate library called ReactDOM. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, react-native, react-art, react-canvas, and react-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.

    To build more environments that React can render to, React team planned to split the main React package into two: react and react-dom. This paves the way to writing components that can be shared between the web version of React and React Native.

[↑] Back to top

98. How to use React label element?

    If you try to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that attribute and prints a warning to the console.

    <label for={'user'}>{'User'}</label>
    <input type={'text'} id={'user'} />

    Since for is a reserved keyword in JavaScript, use htmlFor instead.

    <label htmlFor={'user'}>{'User'}</label>
    <input type={'text'} id={'user'} />

[↑] Back to top

99. How to combine multiple inline style objects?

    You can use spread operator in regular React:

    <button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
      {"Submit"}
    </button>

    If you're using React Native then you can use the array notation:

    <button style={[styles.panel.button, styles.panel.submitButton]}>
      {"Submit"}
    </button>

[↑] Back to top

100. How to re-render the view when the browser is resized?

     You can listen to the resize event in componentDidMount() and then update the dimensions (width and height). You should remove the listener in componentWillUnmount() method.

     class WindowDimensions extends React.Component {
       constructor(props) {
         super(props);
         this.updateDimensions = this.updateDimensions.bind(this);
       }
     
       componentWillMount() {
         this.updateDimensions();
       }
     
       componentDidMount() {
         window.addEventListener("resize", this.updateDimensions);
       }
     
       componentWillUnmount() {
         window.removeEventListener("resize", this.updateDimensions);
       }
     
       updateDimensions() {
         this.setState({
           width: window.innerWidth,
           height: window.innerHeight,
         });
       }
     
       render() {
         return (
           <span>
             {this.state.width} x {this.state.height}
           </span>
         );
       }
     }

[↑] Back to top

101. What is the difference between setState() and replaceState() methods?

     When you use setState() the current and previous states are merged. replaceState() throws out the current state, and replaces it with only what you provide. Usually setState() is used unless you really need to remove all previous keys for some reason. You can also set state to false/null in setState() instead of using replaceState().

[↑] Back to top

102. How to listen to state changes?

     The componentDidUpdate lifecycle method will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.

     componentDidUpdate(object prevProps, object prevState)

     Note: The previous releases of ReactJS also uses componentWillUpdate(object nextProps, object nextState) for state changes. It has been deprecated in latest releases.

[↑] Back to top

103. What is the recommended approach of removing an array element in React state?

     The better approach is to use Array.prototype.filter() method.

     For example, let's create a removeItem() method for updating the state.

     removeItem(index) {
       this.setState({
         data: this.state.data.filter((item, i) => i !== index)
       })
     }

[↑] Back to top

104. Is it possible to use React without rendering HTML?

     It is possible with latest version (>=16.2). Below are the possible options:

     render() {
       return false
     }

     render() {
       return null
     }

     render() {
       return []
     }

     render() {
       return <React.Fragment></React.Fragment>
     }

     render() {
       return <></>
     }

     Returning undefined won't work.

[↑] Back to top

105. How to pretty print JSON with React?

     We can use <pre> tag so that the formatting of the JSON.stringify() is retained:

     const data = { name: "John", age: 42 };
     
     class User extends React.Component {
       render() {
         return <pre>{JSON.stringify(data, null, 2)}</pre>;
       }
     }
     
     React.render(<User />, document.getElementById("container"));

[↑] Back to top

106. Why you can't update props in React?

     The React philosophy is that props should be immutable and top-down. This means that a parent can send any prop values to a child, but the child can't modify received props.

[↑] Back to top

107. How to focus an input element on page load?

     You can do it by creating ref for input element and using it in componentDidMount():

     class App extends React.Component {
       componentDidMount() {
         this.nameInput.focus();
       }
     
       render() {
         return (
           <div>
             <input defaultValue={"Won't focus"} />
             <input
               ref={(input) => (this.nameInput = input)}
               defaultValue={"Will focus"}
             />
           </div>
         );
       }
     }
     
     ReactDOM.render(<App />, document.getElementById("app"));

[↑] Back to top

108. What are the possible ways of updating objects in state?

     1. Calling setState() with an object to merge with state:

        • Using Object.assign() to create a copy of the object:

          const user = Object.assign({}, this.state.user, { age: 42 });
          this.setState({ user });

        • Using spread operator:

          const user = { ...this.state.user, age: 42 };
          this.setState({ user });

     2. Calling setState() with a function:

        this.setState((prevState) => ({
          user: {
            ...prevState.user,
            age: 42,
          },
        }));

[↑] Back to top

110. How can we find the version of React at runtime in the browser?

     You can use React.version to get the version.

     const REACT_VERSION = React.version;
     
     ReactDOM.render(
       <div>{`React version: ${REACT_VERSION}`}</div>,
       document.getElementById("app")
     );

[↑] Back to top

111. What are the approaches to include polyfills in your create-react-app?

     There are approaches to include polyfills in create-react-app,

     1. Manual import from core-js:

        Create a file called (something like) polyfills.js and import it into root index.js file. Run npm install core-js or yarn add core-js and import your specific required features.

        import "core-js/fn/array/find";
        import "core-js/fn/array/includes";
        import "core-js/fn/number/is-nan";

     2. Using Polyfill service:

        Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to index.html:

        <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.includes"></script>

        In the above script we had to explicitly request the Array.prototype.includes feature as it is not included in the default feature set.

[↑] Back to top

112. How to use https instead of http in create-react-app?

     You just need to use HTTPS=true configuration. You can edit your package.json scripts section:

     "scripts": {
       "start": "set HTTPS=true && react-scripts start"
     }

     or just run set HTTPS=true && npm start

[↑] Back to top

113. How to avoid using relative path imports in create-react-app?

     Create a file called .env in the project root and write the import path:

     NODE_PATH=src/app

     After that restart the development server. Now you should be able to import anything inside src/app without relative paths.

[↑] Back to top

114. How to add Google Analytics for React Router?

     Add a listener on the history object to record each page view:

     history.listen(function (location) {
       window.ga("set", "page", location.pathname + location.search);
       window.ga("send", "pageview", location.pathname + location.search);
     });

[↑] Back to top

115. How to update a component every second?

     You need to use setInterval() to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.

     componentDidMount() {
       this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
     }
     
     componentWillUnmount() {
       clearInterval(this.interval)
     }

[↑] Back to top

116. How do you apply vendor prefixes to inline styles in React?

     React does not apply vendor prefixes automatically. You need to add vendor prefixes manually.

     <div
       style={{
         transform: "rotate(90deg)",
         WebkitTransform: "rotate(90deg)", // note the capital 'W' here
         msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix
       }}
     />

[↑] Back to top

117. How to import and export components using React and ES6?

     You should use default for exporting the components

     import React from "react";
     import User from "user";
     
     export default class MyProfile extends React.Component {
       render() {
         return <User type="customer">//...</User>;
       }
     }

     With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.

[↑] Back to top

119. Why is a component constructor called only once?

     React's reconciliation algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.

[↑] Back to top

120. How to define constants in React?

     You can use ES7 static field to define constant.

     class MyComponent extends React.Component {
       static DEFAULT_PAGINATION = 10;
     }

     Static fields are part of the Class Fields stage 3 proposal.

[↑] Back to top

121. How to programmatically trigger click event in React?

     You could use the ref prop to acquire a reference to the underlying HTMLInputElement object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the HTMLElement.click method.

     This can be done in two steps:

     1. Create ref in render method:

        <input ref={(input) => (this.inputElement = input)} />

     2. Apply click event in your event handler:

        this.inputElement.click();

[↑] Back to top

122. Is it possible to use async/await in plain React?

     If you want to use async/await in React, you will need Babel and transform-async-to-generator plugin. React Native ships with Babel and a set of transforms.

[↑] Back to top

123. What are the common folder structures for React?

     There are two common practices for React project file structure.

     1. Grouping by features or routes:

        One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.

        common/
        ├─ Avatar.js
        ├─ Avatar.css
        ├─ APIUtils.js
        └─ APIUtils.test.js
        feed/
        ├─ index.js
        ├─ Feed.js
        ├─ Feed.css
        ├─ FeedStory.js
        ├─ FeedStory.test.js
        └─ FeedAPI.js
        profile/
        ├─ index.js
        ├─ Profile.js
        ├─ ProfileHeader.js
        ├─ ProfileHeader.css
        └─ ProfileAPI.js

     2. Grouping by file type:

        Another popular way to structure projects is to group similar files together.

        api/
        ├─ APIUtils.js
        ├─ APIUtils.test.js
        ├─ ProfileAPI.js
        └─ UserAPI.js
        components/
        ├─ Avatar.js
        ├─ Avatar.css
        ├─ Feed.js
        ├─ Feed.css
        ├─ FeedStory.js
        ├─ FeedStory.test.js
        ├─ Profile.js
        ├─ ProfileHeader.js
        └─ ProfileHeader.css

[↑] Back to top

124. What are the popular packages for animation?

     React Transition Group and React Motion are popular animation packages in React ecosystem.

[↑] Back to top

125. What is the benefit of styles modules?

     It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.

     For example, these styles could be extracted into a separate component:

     export const colors = {
       white,
       black,
       blue,
     };
     
     export const space = [0, 8, 16, 32, 64];

     And then imported individually in other components:

     import { space, colors } from "./styles";

[↑] Back to top

126. What are the popular React-specific linters?

     ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.

     Another popular plugin is eslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text and tabindex, for example, will not be picked up by regular plugins.

[↑] Back to top

127. How to make AJAX call and in which component lifecycle methods should I make an AJAX call?

     You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in fetch. You should fetch data in the componentDidMount() lifecycle method. This is so you can use setState() to update your component when the data is retrieved.

     For example, the employees list fetched from API and set local state:

     class MyComponent extends React.Component {
       constructor(props) {
         super(props);
         this.state = {
           employees: [],
           error: null,
         };
       }
     
       componentDidMount() {
         fetch("https://api.example.com/items")
           .then((res) => res.json())
           .then(
             (result) => {
               this.setState({
                 employees: result.employees,
               });
             },
             (error) => {
               this.setState({ error });
             }
           );
       }
     
       render() {
         const { error, employees } = this.state;
         if (error) {
           return <div>Error: {error.message}</div>;
         } else {
           return (
             <ul>
               {employees.map((employee) => (
                 <li key={employee.name}>
                   {employee.name}-{employee.experience}
                 </li>
               ))}
             </ul>
           );
         }
       }
     }

[↑] Back to top

128. What are render props?

     Render Props is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.

     <DataProvider render={(data) => <h1>{`Hello ${data.target}`}</h1>} />

     Libraries such as React Router and DownShift are using this pattern.

React Router

[↑] Back to top

129. What is React Router?

     React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.

[↑] Back to top

130. How React Router is different from history library?

     React Router is a wrapper around the history library which handles interaction with the browser's window.history with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.

[↑] Back to top

131. What are the <Router> components of React Router v4?

     React Router v4 provides below 3 <Router> components:

     1. <BrowserRouter>
     2. <HashRouter>
     3. <MemoryRouter>

     The above components will create browser, hash, and memory history instances. React Router v4 makes the properties and methods of the history instance associated with your router available through the context in the router object.

[↑] Back to top

132. What is the purpose of push() and replace() methods of history?

     A history instance has two methods for navigation purpose.

     1. push()
     2. replace()

     If you think of the history as an array of visited locations, push() will add a new location to the array and replace() will replace the current location in the array with the new one.

[↑] Back to top

133. How do you programmatically navigate using React Router v4?

     There are three different ways to achieve programmatic routing/navigation within components.

     1. Using the withRouter() higher-order function:

        The withRouter() higher-order function will inject the history object as a prop of the component. This object provides push() and replace() methods to avoid the usage of context.

        import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
        
        const Button = withRouter(({ history }) => (
          <button
            type="button"
            onClick={() => {
              history.push("/new-location");
            }}
          >
            {"Click Me!"}
          </button>
        ));

     2. Using <Route> component and render props pattern:

        The <Route> component passes the same props as withRouter(), so you will be able to access the history methods through the history prop.

        import { Route } from "react-router-dom";
        
        const Button = () => (
          <Route
            render={({ history }) => (
              <button
                type="button"
                onClick={() => {
                  history.push("/new-location");
                }}
              >
                {"Click Me!"}
              </button>
            )}
          />
        );

     3. Using context:

        This option is not recommended and treated as unstable API.

        const Button = (props, context) => (
          <button
            type="button"
            onClick={() => {
              context.history.push("/new-location");
            }}
          >
            {"Click Me!"}
          </button>
        );
        
        Button.contextTypes = {
          history: React.PropTypes.shape({
            push: React.PropTypes.func.isRequired,
          }),
        };

[↑] Back to top

134. How to get query parameters in React Router v4?

     The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.

     const queryString = require("query-string");
     const parsed = queryString.parse(props.location.search);

     You can also use URLSearchParams if you want something native:

     const params = new URLSearchParams(props.location.search);
     const foo = params.get("name");

     You should use a polyfill for IE11.

[↑] Back to top

135. Why you get "Router may have only one child element" warning?

     You have to wrap your Route's in a <Switch> block because <Switch> is unique in that it renders a route exclusively.

     At first you need to add Switch to your imports:

     import { Switch, Router, Route } from "react-router";

     Then define the routes within <Switch> block:

     <Router>
       <Switch>
         <Route {/* ... */} />
         <Route {/* ... */} />
       </Switch>
     </Router>

[↑] Back to top

136. How to pass params to history.push method in React Router v4?

     While navigating you can pass props to the history object:

     this.props.history.push({
       pathname: "/template",
       search: "?name=sudheer",
       state: { detail: response.data },
     });

     The search property is used to pass query params in push() method.

[↑] Back to top

137. How to implement default or NotFound page?

     A <Switch> renders the first child <Route> that matches. A <Route> with no path always matches. So you just need to simply drop path attribute as below

     <Switch>
       <Route exact path="/" component={Home} />
       <Route path="/user" component={User} />
       <Route component={NotFound} />
     </Switch>

[↑] Back to top

138. How to get history on React Router v4?

     Below are the list of steps to get history object on React Router v4,

     1. Create a module that exports a history object and import this module across the project.

        For example, create history.js file:

        import { createBrowserHistory } from "history";
        
        export default createBrowserHistory({
          /* pass a configuration object here if needed */
        });

     2. You should use the <Router> component instead of built-in routers. Imported the above history.js inside index.js file:

        import { Router } from "react-router-dom";
        import history from "./history";
        import App from "./App";
        
        ReactDOM.render(
          <Router history={history}>
            <App />
          </Router>,
          holder
        );

     3. You can also use push method of history object similar to built-in history object:

        // some-other-file.js
        import history from "./history";
        
        history.push("/go-here");

[↑] Back to top

139. How to perform automatic redirect after login?

     The react-router package provides <Redirect> component in React Router. Rendering a <Redirect> will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.

     import React, { Component } from "react";
     import { Redirect } from "react-router";
     
     export default class LoginComponent extends Component {
       render() {
         if (this.state.isLoggedIn === true) {
           return <Redirect to="/your/redirect/page" />;
         } else {
           return <div>{"Login Please"}</div>;
         }
       }
     }

React Internationalization

[↑] Back to top

140. What is React Intl?

     The React Intl library makes internalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of FormatJS which provides bindings to React via its components and API.

[↑] Back to top

141. What are the main features of React Intl?

     Below are the main features of React Intl,

     1. Display numbers with separators.
     2. Display dates and times correctly.
     3. Display dates relative to "now".
     4. Pluralize labels in strings.
     5. Support for 150+ languages.
     6. Runs in the browser and Node.
     7. Built on standards.

[↑] Back to top

142. What are the two ways of formatting in React Intl?

     The library provides two ways to format strings, numbers, and dates:

     1. Using react components:

        <FormattedMessage
          id={"account"}
          defaultMessage={"The amount is less than minimum balance."}
        />

2. **Using an API:**

     ```jsx
     const messages = defineMessages({
       accountMessage: {
         id: 'account',
         defaultMessage: 'The amount is less than minimum balance.',
       }
     })

     formatMessage(messages.accountMessage)
     ```

[↑] Back to top

143. How to use <FormattedMessage> as placeholder using React Intl?

     The <Formatted... /> components from react-intl return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API formatMessage(). You can inject the intl object into your component using injectIntl() higher-order component and then format the message using formatMessage() available on that object.

     import React from "react";
     import { injectIntl, intlShape } from "react-intl";
     
     const MyComponent = ({ intl }) => {
       const placeholder = intl.formatMessage({ id: "messageId" });
       return <input placeholder={placeholder} />;
     };
     
     MyComponent.propTypes = {
       intl: intlShape.isRequired,
     };
     
     export default injectIntl(MyComponent);

[↑] Back to top

144. How to access current locale with React Intl?

     You can get the current locale in any component of your application using injectIntl():

     import { injectIntl, intlShape } from "react-intl";
     
     const MyComponent = ({ intl }) => (
       <div>{`The current locale is ${intl.locale}`}</div>
     );
     
     MyComponent.propTypes = {
       intl: intlShape.isRequired,
     };
     
     export default injectIntl(MyComponent);

[↑] Back to top

145. How to format date using React Intl?

     The injectIntl() higher-order component will give you access to the formatDate() method via the props in your component. The method is used internally by instances of FormattedDate and it returns the string representation of the formatted date.

     import { injectIntl, intlShape } from "react-intl";
     
     const stringDate = this.props.intl.formatDate(date, {
       year: "numeric",
       month: "numeric",
       day: "numeric",
     });
     
     const MyComponent = ({ intl }) => (
       <div>{`The formatted date is ${stringDate}`}</div>
     );
     
     MyComponent.propTypes = {
       intl: intlShape.isRequired,
     };
     
     export default injectIntl(MyComponent);

React Testing

[↑] Back to top

146. What is Shallow Renderer in React testing?

     Shallow rendering is useful for writing unit test cases in React. It lets you render a component one level deep and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.

     For example, if you have the following component:

     function MyComponent() {
       return (
         <div>
           <span className={"heading"}>{"Title"}</span>
           <span className={"description"}>{"Description"}</span>
         </div>
       );
     }

     Then you can assert as follows:

     import ShallowRenderer from "react-test-renderer/shallow";
     
     // in your test
     const renderer = new ShallowRenderer();
     renderer.render(<MyComponent />);
     
     const result = renderer.getRenderOutput();
     
     expect(result.type).toBe("div");
     expect(result.props.children).toEqual([
       <span className={"heading"}>{"Title"}</span>,
       <span className={"description"}>{"Description"}</span>,
     ]);

[↑] Back to top

147. What is TestRenderer package in React?

     This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or jsdom.

     import TestRenderer from "react-test-renderer";
     
     const Link = ({ page, children }) => <a href={page}>{children}</a>;
     
     const testRenderer = TestRenderer.create(
       <Link page={"https://www.facebook.com/"}>{"Facebook"}</Link>
     );
     
     console.log(testRenderer.toJSON());
     // {
     //   type: 'a',
     //   props: { href: 'https://www.facebook.com/' },
     //   children: [ 'Facebook' ]
     // }

[↑] Back to top

148. What is the purpose of ReactTestUtils package?

     ReactTestUtils are provided in the with-addons package and allow you to perform actions against a simulated DOM for the purpose of unit testing.

[↑] Back to top

149. What is Jest?

     Jest is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a jsdom environment. It's often used for testing components.

[↑] Back to top

150. What are the advantages of Jest over Jasmine?

     There are couple of advantages compared to Jasmine:

     • Automatically finds tests to execute in your source code.
     • Automatically mocks dependencies when running your tests.
     • Allows you to test asynchronous code synchronously.
     • Runs your tests with a fake DOM implementation (via jsdom) so that your tests can be run on the command line.
     • Runs tests in parallel processes so that they finish sooner.

[↑] Back to top

151. Give a simple example of Jest test case

     Let's write a test for a function that adds two numbers in sum.js file:

     const sum = (a, b) => a + b;
     
     export default sum;

     Create a file named sum.test.js which contains actual test:

     import sum from "./sum";
     
     test("adds 1 + 2 to equal 3", () => {
       expect(sum(1, 2)).toBe(3);
     });

     And then add the following section to your package.json:

     {
       "scripts": {
         "test": "jest"
       }
     }

     Finally, run yarn test or npm test and Jest will print a result:

     $ yarn test
     PASS ./sum.test.js
     ✓ adds 1 + 2 to equal 3 (2ms)

React Redux

[↑] Back to top

152. What is flux?

     Flux is an application design paradigm used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.

     The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:

     

[↑] Back to top

153. What is Redux?

     Redux is a predictable state container for JavaScript apps based on the Flux design pattern. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies.

[↑] Back to top

154. What are the core principles of Redux?

     Redux follows three fundamental principles:

     1. Single source of truth: The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
     2. State is read-only: The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
     3. Changes are made with pure functions: To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.

[↑] Back to top

155. What are the downsides of Redux compared to Flux?

     Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:

     1. You will need to learn to avoid mutations: Flux is un-opinionated about mutating data, but Redux doesn't like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like redux-immutable-state-invariant, Immutable.js, or instructing your team to write non-mutating code.
     2. You're going to have to carefully pick your packages: While Flux explicitly doesn't try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a rich ecosystem.
     3. There is no nice Flow integration yet: Flux currently lets you do very impressive static type checks which Redux doesn't support yet.

[↑] Back to top

156. What is the difference between mapStateToProps() and mapDispatchToProps()?

     mapStateToProps() is a utility which helps your component get updated state (which is updated by some other components):

     const mapStateToProps = (state) => {
       return {
         todos: getVisibleTodos(state.todos, state.visibilityFilter),
       };
     };

     mapDispatchToProps() is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):

     const mapDispatchToProps = (dispatch) => {
       return {
         onTodoClick: (id) => {
           dispatch(toggleTodo(id));
         },
       };
     };

     Recommend always using the “object shorthand” form for the mapDispatchToProps

     Redux wrap it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.

     const mapDispatchToProps = {
       onTodoClick,
     };

[↑] Back to top

157. Can I dispatch an action in reducer?

     Dispatching an action within a reducer is an anti-pattern. Your reducer should be without side effects, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.

[↑] Back to top

158. How to access Redux store outside a component?

     You just need to export the store from the module where it created with createStore(). Also, it shouldn't pollute the global window object.

     store = createStore(myReducer);
     
     export default store;

[↑] Back to top

159. What are the drawbacks of MVW pattern?

     1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
     2. Due to circular dependencies, a complicated model was created around models and views.
     3. Lot of data changes happens for collaborative applications(like Google Docs).
     4. No way to do undo (travel back in time) easily without adding so much extra code.

[↑] Back to top

160. Are there any similarities between Redux and RxJS?

     These libraries are very different for very different purposes, but there are some vague similarities.

     Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.

[↑] Back to top

161. How to dispatch an action on load?

     You can dispatch an action in componentDidMount() method and in render() method you can verify the data.

     class App extends Component {
       componentDidMount() {
         this.props.fetchData();
       }
     
       render() {
         return this.props.isLoaded ? (
           <div>{"Loaded"}</div>
         ) : (
           <div>{"Not Loaded"}</div>
         );
       }
     }
     
     const mapStateToProps = (state) => ({
       isLoaded: state.isLoaded,
     });
     
     const mapDispatchToProps = { fetchData };
     
     export default connect(mapStateToProps, mapDispatchToProps)(App);

[↑] Back to top

162. How to use connect() from React Redux?

     You need to follow two steps to use your store in your container:

     1. Use mapStateToProps(): It maps the state variables from your store to the props that you specify.

     2. Connect the above props to your container: The object returned by the mapStateToProps function is connected to the container. You can import connect() from react-redux.

        import React from "react";
        import { connect } from "react-redux";
        
        class App extends React.Component {
          render() {
            return <div>{this.props.containerData}</div>;
          }
        }
        
        function mapStateToProps(state) {
          return { containerData: state.data };
        }
        
        export default connect(mapStateToProps)(App);

[↑] Back to top

163. How to reset state in Redux?

     You need to write a root reducer in your application which delegate handling the action to the reducer generated by combineReducers().

     For example, let us take rootReducer() to return the initial state after USER_LOGOUT action. As we know, reducers are supposed to return the initial state when they are called with undefined as the first argument, no matter the action.

     const appReducer = combineReducers({
       /* your app's top-level reducers */
     });
     
     const rootReducer = (state, action) => {
       if (action.type === "USER_LOGOUT") {
         state = undefined;
       }
     
       return appReducer(state, action);
     };

     In case of using redux-persist, you may also need to clean your storage. redux-persist keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.

     const appReducer = combineReducers({
       /* your app's top-level reducers */
     });
     
     const rootReducer = (state, action) => {
       if (action.type === "USER_LOGOUT") {
         Object.keys(state).forEach((key) => {
           storage.removeItem(`persist:${key}`);
         });
     
         state = undefined;
       }
     
       return appReducer(state, action);
     };

[↑] Back to top

164. Whats the purpose of at symbol in the Redux connect decorator?

     The @ symbol is in fact a JavaScript expression used to signify decorators. Decorators make it possible to annotate and modify classes and properties at design time.

     Let's take an example setting up Redux without and with a decorator.

     • Without decorator:

       import React from "react";
       import * as actionCreators from "./actionCreators";
       import { bindActionCreators } from "redux";
       import { connect } from "react-redux";
       
       function mapStateToProps(state) {
         return { todos: state.todos };
       }
       
       function mapDispatchToProps(dispatch) {
         return { actions: bindActionCreators(actionCreators, dispatch) };
       }
       
       class MyApp extends React.Component {
         // ...define your main app here
       }
       
       export default connect(mapStateToProps, mapDispatchToProps)(MyApp);

     • With decorator:

       import React from "react";
       import * as actionCreators from "./actionCreators";
       import { bindActionCreators } from "redux";
       import { connect } from "react-redux";
       
       function mapStateToProps(state) {
         return { todos: state.todos };
       }
       
       function mapDispatchToProps(dispatch) {
         return { actions: bindActionCreators(actionCreators, dispatch) };
       }
       
       @connect(mapStateToProps, mapDispatchToProps)
       export default class MyApp extends React.Component {
         // ...define your main app here
       }

     The above examples are almost similar except the usage of decorator. The decorator syntax isn't built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.

[↑] Back to top

165. What is the difference between React context and React Redux?

     You can use Context in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for.

     Whereas Redux is much more powerful and provides a large number of features that the Context API doesn't provide. Also, React Redux uses context internally but it doesn't expose this fact in the public API.

[↑] Back to top

166. Why are Redux state functions called reducers?

     Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could reduce a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.

[↑] Back to top

167. How to make AJAX request in Redux?

     You can use redux-thunk middleware which allows you to define async actions.

     Let's take an example of fetching specific account as an AJAX call using fetch API:

     export function fetchAccount(id) {
       return (dispatch) => {
         dispatch(setLoadingAccountState()); // Show a loading spinner
         fetch(`/account/${id}`, (response) => {
           dispatch(doneFetchingAccount()); // Hide loading spinner
           if (response.status === 200) {
             dispatch(setAccount(response.json)); // Use a normal function to set the received state
           } else {
             dispatch(someError);
           }
         });
       };
     }
     
     function setAccount(data) {
       return { type: "SET_Account", data: data };
     }

[↑] Back to top

168. Should I keep all component's state in Redux store?

     Keep your data in the Redux store, and the UI related state internally in the component.

[↑] Back to top

169. What is the proper way to access Redux store?

     The best way to access your store in a component is to use the connect() function, that creates a new component that wraps around your existing one. This pattern is called Higher-Order Components, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.

     Let's take an example of <FilterLink> component using connect:

     import { connect } from "react-redux";
     import { setVisibilityFilter } from "../actions";
     import Link from "../components/Link";
     
     const mapStateToProps = (state, ownProps) => ({
       active: ownProps.filter === state.visibilityFilter,
     });
     
     const mapDispatchToProps = (dispatch, ownProps) => ({
       onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
     });
     
     const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);
     
     export default FilterLink;

     Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using connect() over accessing the store directly (using context API).

     class MyComponent {
       someMethod() {
         doSomethingWith(this.context.store);
       }
     }

[↑] Back to top

170. What is the difference between component and container in React Redux?

     Component is a class or function component that describes the presentational part of your application.

     Container is an informal term for a component that is connected to a Redux store. Containers subscribe to Redux state updates and dispatch actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.

[↑] Back to top

171. What is the purpose of the constants in Redux?

     Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a ReferenceError immediately.

     Normally we will save them in a single file (constants.js or actionTypes.js).

     export const ADD_TODO = "ADD_TODO";
     export const DELETE_TODO = "DELETE_TODO";
     export const EDIT_TODO = "EDIT_TODO";
     export const COMPLETE_TODO = "COMPLETE_TODO";
     export const COMPLETE_ALL = "COMPLETE_ALL";
     export const CLEAR_COMPLETED = "CLEAR_COMPLETED";

     In Redux, you use them in two places:

     1. During action creation:

        Let's take actions.js:

        import { ADD_TODO } from "./actionTypes";
        
        export function addTodo(text) {
          return { type: ADD_TODO, text };
        }

     2. In reducers:

        Let's create reducer.js:

        import { ADD_TODO } from "./actionTypes";
        
        export default (state = [], action) => {
          switch (action.type) {
            case ADD_TODO:
              return [
                ...state,
                {
                  text: action.text,
                  completed: false,
                },
              ];
            default:
              return state;
          }
        };

[↑] Back to top

172. What are the different ways to write mapDispatchToProps()?

     There are a few ways of binding action creators to dispatch() in mapDispatchToProps().

     Below are the possible options:

     const mapDispatchToProps = (dispatch) => ({
       action: () => dispatch(action()),
     });

     const mapDispatchToProps = (dispatch) => ({
       action: bindActionCreators(action, dispatch),
     });

     const mapDispatchToProps = { action };

     The third option is just a shorthand for the first one.

[↑] Back to top

173. What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?

     If the ownProps parameter is specified, React Redux will pass the props that were passed to the component into your connect functions. So, if you use a connected component:

     import ConnectedComponent from "./containers/ConnectedComponent";
     
     <ConnectedComponent user={"john"} />;

     The ownProps inside your mapStateToProps() and mapDispatchToProps() functions will be an object:

     {
       user: "john";
     }

     You can use this object to decide what to return from those functions.

[↑] Back to top

174. How to structure Redux top level directories?

     Most of the applications has several top-level directories as below:

     1. Components: Used for dumb components unaware of Redux.
     2. Containers: Used for smart components connected to Redux.
     3. Actions: Used for all action creators, where file names correspond to part of the app.
     4. Reducers: Used for all reducers, where files name correspond to state key.
     5. Store: Used for store initialization.

     This structure works well for small and medium size apps.

[↑] Back to top

175. What is redux-saga?

     redux-saga is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.

     It is available in NPM:

     $ npm install --save redux-saga

[↑] Back to top

176. What is the mental model of redux-saga?

     Saga is like a separate thread in your application, that's solely responsible for side effects. redux-saga is a redux middleware, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.

[↑] Back to top

177. What are the differences between call() and put() in redux-saga?

     Both call() and put() are effect creator functions. call() function is used to create effect description, which instructs middleware to call the promise. put() function creates an effect, which instructs middleware to dispatch an action to the store.

     Let's take example of how these effects work for fetching particular user data.

     function* fetchUserSaga(action) {
       // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
       // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
       const userData = yield call(api.fetchUser, action.userId);
     
       // Instructing middleware to dispatch corresponding action.
       yield put({
         type: "FETCH_USER_SUCCESS",
         userData,
       });
     }

[↑] Back to top

178. What is Redux Thunk?

     Redux Thunk middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch() and getState() as parameters.

[↑] Back to top

179. What are the differences between redux-saga and redux-thunk?

     Both Redux Thunk and Redux Saga take care of dealing with side effects. In most of the scenarios, Thunk uses Promises to deal with them, whereas Saga uses Generators. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.

[↑] Back to top

180. What is Redux DevTools?

     Redux DevTools is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.

[↑] Back to top

181. What are the features of Redux DevTools?

     Some of the main features of Redux DevTools are below,

     1. Lets you inspect every state and action payload.
     2. Lets you go back in time by cancelling actions.
     3. If you change the reducer code, each staged action will be re-evaluated.
     4. If the reducers throw, you will see during which action this happened, and what the error was.
     5. With persistState() store enhancer, you can persist debug sessions across page reloads.

[↑] Back to top

182. What are Redux selectors and why to use them?

     Selectors are functions that take Redux state as an argument and return some data to pass to the component.

     For example, to get user details from the state:

     const getUserData = (state) => state.user.data;

     These selectors have two main benefits,

     1. The selector can compute derived data, allowing Redux to store the minimal possible state
     2. The selector is not recomputed unless one of its arguments changes

[↑] Back to top

183. What is Redux Form?

     Redux Form works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.

[↑] Back to top

184. What are the main features of Redux Form?

     Some of the main features of Redux Form are:

     1. Field values persistence via Redux store.
     2. Validation (sync/async) and submission.
     3. Formatting, parsing and normalization of field values.

[↑] Back to top

185. How to add multiple middlewares to Redux?

     You can use applyMiddleware().

     For example, you can add redux-thunk and logger passing them as arguments to applyMiddleware():

     import { createStore, applyMiddleware } from "redux";
     const createStoreWithMiddleware = applyMiddleware(
       ReduxThunk,
       logger
     )(createStore);

[↑] Back to top

186. How to set initial state in Redux?

     You need to pass initial state as second argument to createStore:

     const rootReducer = combineReducers({
       todos: todos,
       visibilityFilter: visibilityFilter,
     });
     
     const initialState = {
       todos: [{ id: 123, name: "example", completed: false }],
     };
     
     const store = createStore(rootReducer, initialState);

[↑] Back to top

187. How Relay is different from Redux?

     Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via GraphQL queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.

188. What is an action in Redux?

     Actions are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.

     For example, let's take an action which represents adding a new todo item:

     {
       type: ADD_TODO,
       text: 'Add todo item'
     }

[↑] Back to top

React Native

[↑] Back to top

188. What is the difference between React Native and React?

     React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.

     React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.

[↑] Back to top

189. How to test React Native apps?

     React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.

[↑] Back to top

190. How to do logging in React Native?

     You can use console.log, console.warn, etc. As of React Native v0.29 you can simply run the following to see logs in the console:

     $ react-native log-ios
     $ react-native log-android

[↑] Back to top

191. How to debug your React Native?

     Follow the below steps to debug React Native app:

     1. Run your application in the iOS simulator.
     2. Press Command + D and a webpage should open up at http://localhost:8081/debugger-ui.
     3. Enable Pause On Caught Exceptions for a better debugging experience.
     4. Press Command + Option + I to open the Chrome Developer tools, or open it via View -> Developer -> Developer Tools.
     5. You should now be able to debug as you normally would.

React supported libraries & Integration

[↑] Back to top

192. What is reselect and how it works?

     Reselect is a selector library (for Redux) which uses memoization concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.

     Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.

[↑] Back to top

193. What is Flow?

     Flow is a static type checker designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving null, unlike most type systems.

[↑] Back to top

194. What is the difference between Flow and PropTypes?

     Flow is a static analysis tool (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.

     PropTypes is a basic type checker (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.

[↑] Back to top

195. How to use Font Awesome icons in React?

     The below steps followed to include Font Awesome in React:

     1. Install font-awesome:

        $ npm install --save font-awesome

     2. Import font-awesome in your index.js file:

        import "font-awesome/css/font-awesome.min.css";

     3. Add Font Awesome classes in className:

        render() {
          return <div><i className={'fa fa-spinner'} /></div>
        }

[↑] Back to top

196. What is React Dev Tools?

     React Developer Tools let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).

     The official extensions available for different browsers or environments.

     1. Chrome extension
     2. Firefox extension
     3. Standalone app (Safari, React Native, etc)

[↑] Back to top

197. Why is DevTools not loading in Chrome for local files?

     If you opened a local HTML file in your browser (file://...) then you must first open Chrome Extensions and check Allow access to file URLs.

[↑] Back to top

198. How to use Polymer in React?

     You need to follow below steps to use Polymer in React,

     1. Create a Polymer element:

        <link
          rel="import"
          href="../../bower_components/polymer/polymer.html"
        />;
        Polymer({
          is: "calender-element",
          ready: function () {
            this.textContent = "I am a calender";
          },
        });

     2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the index.html of your React application:

        <link
          rel="import"
          href="./src/polymer-components/calender-element.html"
        />

     3. Use that element in the JSX file:

        import React from "react";
        
        class MyComponent extends React.Component {
          render() {
            return <calender-element />;
          }
        }
        
        export default MyComponent;

[↑] Back to top

199. What are the advantages of React over Vue.js?

     React has the following advantages over Vue.js:

     1. Gives more flexibility in large apps developing.
     2. Easier to test.
     3. Suitable for mobile apps creating.
     4. More information and solutions available.

Note: The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

[↑] Back to top

200. What is the difference between React and Angular?

     Let's see the difference between React and Angular in a table format.

     React	Angular
     React is a library and has only the View layer	Angular is a framework and has complete MVC functionality
     React handles rendering on the server side	AngularJS renders only on the client side but Angular 2 and above renders on the server side
     React uses JSX that looks like HTML in JS which can be confusing	Angular follows the template approach for HTML, which makes code shorter and easy to understand
     React Native, which is a React type to build mobile applications are faster and more stable	Ionic, Angular's mobile native app is relatively less stable and slower
     In React, data flows only in one way and hence debugging is easy	In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult

Note: The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

[↑] Back to top

201. Why React tab is not showing up in DevTools?

     When the page loads, React DevTools sets a global named __REACT_DEVTOOLS_GLOBAL_HOOK__, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.

[↑] Back to top

202. What are Styled Components?

     styled-components is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.

[↑] Back to top

203. Give an example of Styled Components?

     Lets create <Title> and <Wrapper> components with specific styles for each.

     import React from "react";
     import styled from "styled-components";
     
     // Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
     const Title = styled.h1`
       font-size: 1.5em;
       text-align: center;
       color: palevioletred;
     `;
     
     // Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
     const Wrapper = styled.section`
       padding: 4em;
       background: papayawhip;
     `;

     These two variables, Title and Wrapper, are now components that you can render just like any other react component.

     <Wrapper>
       <Title>{"Lets start first styled component!"}</Title>
     </Wrapper>

[↑] Back to top

204. What is Relay?

     Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.

[↑] Back to top

205. How to use TypeScript in create-react-app application?

     Starting from react-scripts@2.1.0 or higher, there is a built-in support for typescript. i.e, create-react-app now supports typescript natively. You can just pass --typescript option as below

     npx create-react-app my-app --typescript
     
     # or
     
     yarn create react-app my-app --typescript

     But for lower versions of react scripts, just supply --scripts-version option as react-scripts-ts while you create a new project. react-scripts-ts is a set of adjustments to take the standard create-react-app project pipeline and bring TypeScript into the mix.

     Now the project layout should look like the following:

     my-app/
     ├─ .gitignore
     ├─ images.d.ts
     ├─ node_modules/
     ├─ public/
     ├─ src/
     │  └─ ...
     ├─ package.json
     ├─ tsconfig.json
     ├─ tsconfig.prod.json
     ├─ tsconfig.test.json
     └─ tslint.json

Miscellaneous

[↑] Back to top

206. What are the main features of Reselect library?

     Let's see the main features of Reselect library,

     1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
     2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
     3. Selectors are composable. They can be used as input to other selectors.

207. Give an example of Reselect usage?

     Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:

     import { createSelector } from "reselect";
     
     const shopItemsSelector = (state) => state.shop.items;
     const taxPercentSelector = (state) => state.shop.taxPercent;
     
     const subtotalSelector = createSelector(shopItemsSelector, (items) =>
       items.reduce((acc, item) => acc + item.value, 0)
     );
     
     const taxSelector = createSelector(
       subtotalSelector,
       taxPercentSelector,
       (subtotal, taxPercent) => subtotal * (taxPercent / 100)
     );
     
     export const totalSelector = createSelector(
       subtotalSelector,
       taxSelector,
       (subtotal, tax) => ({ total: subtotal + tax })
     );
     
     let exampleState = {
       shop: {
         taxPercent: 8,
         items: [
           { name: "apple", value: 1.2 },
           { name: "orange", value: 0.95 },
         ],
       },
     };
     
     console.log(subtotalSelector(exampleState)); // 2.15
     console.log(taxSelector(exampleState)); // 0.172
     console.log(totalSelector(exampleState)); // { total: 2.322 }

[↑] Back to top

209. Does the statics object work with ES6 classes in React?

     No, statics only works with React.createClass():

     someComponent = React.createClass({
       statics: {
         someMethod: function () {
           // ..
         },
       },
     });

     But you can write statics inside ES6+ classes as below,

     class Component extends React.Component {
       static propTypes = {
         // ...
       };
     
       static someMethod() {
         // ...
       }
     }

     or writing them outside class as below,

     class Component extends React.Component {
        ....
     }
     
     Component.propTypes = {...}
     Component.someMethod = function(){....}

[↑] Back to top

210. Can Redux only be used with React?

     Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.