ReactJs interview Questions Part 2

11.  Why should we not update the state directly?

If you try to update the 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.

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'))

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

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

                                i.            In HTML, the event name usually represents in lowercase as a convention:

<button onclick='activateLasers()'>

Whereas in React it follows camelCase convention:

<button onClick={activateLasers}>

                              ii.            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.')

}

                            iii.            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)

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

There are 3 possible ways to achieve this:

                                i.            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.

           ii.     class Foo extends Component {

         iii.       constructor(props) {

           iv.         super(props);

            v.         this.handleClick = this.handleClick.bind(this);

           vi.       }

         vii.       handleClick() {

        viii.         console.log('Click happened');

           ix.       }

            x.       render() {

           xi.         return <button onClick={this.handleClick}>Click Me</button>;

         xii.       }

}

                         xiii.            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.

         xiv.     handleClick = () => {

           xv.       console.log('this is:', this)

}

<button onClick={this.handleClick}>

  {'Click me'}

</button>

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

        xvii.     handleClick() {

      xviii.         console.log('Click happened');

         xix.     }

           xx.     render() {

         xxi.         return <button onClick={() => this.handleClick()}>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.

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 arrow function

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

handleClick = (id) => () => {

    console.log("Hello, your ticket number is", id)

};

16.What are synthetic events in React?

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

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>

}

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:

                                 i.            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.

                               ii.            If you extract list item as separate component then apply keys on list component instead of li tag.

                             iii.            There will be a warning message in the console if the key prop is not present on list items.

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.

20.    How to create refs?

There are two approaches

                                i.            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.

                               ii.     class MyComponent extends React.Component {

                             iii.       constructor(props) {

                               iv.         super(props)

                                v.         this.myRef = React.createRef()

                               vi.       }

                             vii.       render() {

                            viii.         return <div ref={this.myRef} />

                                   ix.       }

}

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

                           xi.     class SearchBar extends Component {

                         xii.        constructor(props) {

                            xiii.           super(props);

                         xiv.           this.txtSearch = null;

                               xv.           this.state = { term: '' };

                         xvi.           this.setInputSearchRef = e => {

                        xvii.              this.txtSearch = e;

                          xviii.           }

                         xix.        }

                           xx.        onInputChange(event) {

                         xxi.           this.setState({ term: this.txtSearch.value });

                        xxii.        }

                          xxiii.        render() {

                        xxiv.           return (

                         xxv.              <input

                        xxvi.                 value={this.state.term}

                          xxvii.                 onChange={this.onInputChange.bind(this)}

                         xxviii.                 ref={this.setInputSearchRef} />

                        xxix.           );

                             xxx.        }

}

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.

Source : https://github.com/sudheerj/reactjs-interview-questions#table-of-contents.