Top Reactjs Interview Questions and Answers to Get a Job in

Top Reactjs Interview Questions and Answers to Get a Job in

From Lamia Jonas

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Well, well, well. Are you the one who is going to have an interview in the IT company? And are you the one who is going to work with ReactJS? Don’t be nervous, my friend. I gathered here the most important questions that you may be asked during the interview. Some of them are somehow difficult to answer immediately, so better prepare as well as you can and try to remember these questions and answers.

However, I still recommend you to develop your intuition, because most of the tasks you receive you will solve intuitively and rely on your knowledge, not on what you have “memorized” just for the interview.

Let’s start: Top ReactJS interview questions and answers to get a job in a company.

1. What does setState do?

React's setState rebuilds the application and updates the DOM. How does this happen? It's simple. React is very lightweight. It builds a copy of the DOM tree inside itself, and when improving the application, it simply compares the previous version of the DOM with the new one, finds changes, and effectively injects them into the interface.

You don't need to worry about setState: just name this function with a new value as an object:

this.setState ({someField: someValue})

React sees the exact changes, thus preventing unnecessary manipulation of the user interface.

2. What is the difference between an element and a React.JS component?

To say it in simple words: an element is what you want to see on the screen.

A little bit more complicated: an element is an object representation of some user interface.

The component is different. It is a class or function that can take data and return elements (usually using the React JSX element creation technique).

3. When is Class Component better than Functional Component?

Defining a component as a class is a powerful implementation that is needed if the component has state or meaningful methods. In other cases, you can use a functional component: it is less powerful, but simpler.

4. What are refs and what are they themselves?

Let’s suppose that we need to "reach out" to a specific element and use a method. ReactJS refs is just for this. How to use it? Add ref attribute to component for callback. For instance:

class UnControlledForm extends Component {

  handleSubmit = () => {

    console.log ("Input Value:", this.input.value)

  }

  render () {

    return (

      <form onSubmit = {this.handleSubmit}>

        <input

          type = 'text'

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

        <button type = 'submit'> Submit </button>

      </form>

    )

  }

}

It is noteworthy that the input field has a ref attribute, and its value is a function. It receives a DOM element, which is inserted into the instance so that it can then be accessed in the handleSubmit function. In this case, you can work with both the component class and functional components using a closure:

function CustomForm ({handleSubmit}) {

  let inputElement

  return (

    <form onSubmit = {() => handleSubmit (inputElement.value)}>

      <input

        type = 'text'

        ref = {(input) => inputElement = input} />

      <button type = 'submit'> Submit </button>

    </form>

  )

}

5. What is the React key?

I would rather say that it is a real lifesaver, especially in large code. Keys keep track of items that have been changed, added, or removed from the list. This can be done like this:

render () {

  return (

    <ul>

      {this.state.todoItems.map (({task, uid}) => {

        return <li key = {uid}> {task} </li>

      })}

    </ul>

  )

}

Uniqueness is important, because we are talking about matching a new tree with the previous one.

6. What is the difference between controlled and uncontrolled components?

A distinctive feature of React.JS is the management of components and their state.

A controllable component is a component within which React monitors. In the example, username exists in the component state, not in the DOM, and to update the username, we simply use setState:

class ControlledForm extends Component {

  state = {

    username: ''

  }

  updateUsername = (e) => {

    this.setState ({

      username: e.target.value,

    })

  }

  handleSubmit = () => {}

  render () {

    return (

      <form onSubmit = {this.handleSubmit}>

        <input

          type = 'text'

          value = {this.state.username}

          onChange = {this.updateUsername} />

        <button type = 'submit'> Submit </button>

      </form>

    )

  }

}

Uncontrolled components, however, are those in which form data is processed by the DOM. For this, ref is used:

class UnControlledForm extends Component {

  handleSubmit = () => {

    console.log ("Input Value:", this.input.value)

  }

  render () {

    return (

      <form onSubmit = {this.handleSubmit}>

        <input

          type = 'text'

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

        <button type = 'submit'> Submit </button>

      </form>

    )

  }

}

These components are easier to implement because the values ​​are taken from the DOM using refs. Only now it is recommended to pay attention to the controlled components and understand how to work with them. The point is that they support fast field validation, allow you to set the format of the input data and enable/disable buttons, and this is the style of React.

7. Where should AJAX requests be and why?

AJAX requests are made in the componentDidMount method. This is supported by two reasons:

      First, it will improve performance because React can stop and start rendering. Conversely, if you prefer componentWillMount, React will call it whenever it sees the need for it. For AJAX requests, componentWillMount is not the best option.

      Second, componentDidMount ensures that there is a component to update. Otherwise, crashes may fall.

8. What is this shouldComponentUpdate?

I have already told about setState and found out what it does. But what should be done to “protect” certain components from comparison? This is what the shouldComponentUpdate method for.

Ask why is it needed? Well, let's say you know for sure that part of the UI will not change. And why should we run it through comparison if it doesn't make any sense? Returning false from the shouldComponentUpdate method will let React know that the components included in the method do not need to be compared and changed.

9. Describe event handling in React.JS

Cross-browser compatibility is one of the main problems, and to solve it, React handlers are using instances of SyntheticEvent - wrappers over native browser events. These events are artificial and have the same interface as regular ones, only they work equally well in all browsers.

Curiously, React doesn't bind events to child elements. Instead, it is related to top-level events, which frees React from keeping track of listeners when the DOM is updated, and is good for performance.

10. What is the difference between cloneElement and createElement?

      cloneElement is intended for exact copying of an element and binding the necessary parameters to it;

      createElement is what JSX broadcasts, and also what React.JS uses to create new elements that represent the UI.

11. What second argument can be passed to setState?

This is a callback function. It is implemented strictly after setState, when the element is rendered, and is completely optional. It is recommended to give preference to the other method than this function, but it will not be redundant to know about its existence and how it works:

this.setState (

  {username: 'tylermcginnis33'},

  () => console.log ('setState has finished and the component has re-rendered.')

)

12. Point out the error in the given code

this.setState ((prevState, props) => {

  return {

    streak: prevState.streak + props.count

Actually, there are no errors here :). However, this option is used extremely rarely, so probably you’ll have some questions. However, you can pass a setState function to React that takes props and the previous state, and returns a new state, similar to the examples above. This version of the code is optimal if the current state is updated based on the state of the previous one.

Well, that’s it. These were the hardest questions you can face in the interview of react js development company. Believe in yourself and be confident in your answers. Good luck in finding your perfect job!

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