Amazon React JS Interview Questions
Amazon India React JS Interview Questions_

Amazon React JS Interview Questions

On October 15, 2024, Posted by , In Reactjs, With Comments Off on Amazon React JS Interview Questions
Amazon India React JS Interview Questions_

Table of Contents:

When preparing for React JS interviews, particularly with a tech giant like Amazon, it’s essential to grasp both fundamental and advanced concepts of the React library. React, developed by Facebook, is a powerful library used for building user interfaces. Mastering it can set a solid foundation for handling complex web applications. Amazon, known for its high standards in software development, often focuses on assessing candidates’ proficiency in React through a range of questions that test understanding of core principles, performance optimization, and best practices in component-based architecture.

In an Amazon React JS interview, I can expect questions that delve into various aspects of React, from lifecycle methods and state management to more intricate topics like error boundaries and performance tuning. The interviewers aim to evaluate not just technical skills but also problem-solving abilities and how well I can apply React concepts in practical scenarios. Preparing thoroughly by reviewing React’s key features, common pitfalls, and advanced techniques will help me demonstrate a strong command of the framework and its best practices during the React JS interview process.

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Explore: React JSX

1. How to create refs?

In React, refs (short for references) provide a way to access DOM elements or React elements directly in a React component. I use refs to interact with DOM elements that aren’t managed by React’s state system, like focusing on an input or getting the value of a child component. The primary method for creating refs is by using React.createRef() or the useRef() hook in functional components. By creating a ref and attaching it to a JSX element, I can manipulate that element later.

Here’s a simple example:

class MyComponent extends React.Component {  
  constructor(props) {  
    super(props);  
    this.myRef = React.createRef();  
  }  
  componentDidMount() {  
    this.myRef.current.focus();  
  }  
  render() {  
    return <input ref={this.myRef} type="text" />;  
  }  
}

In the example, I create a reference using React.createRef() in the constructor and attach it to the input element using the ref attribute. Then, in componentDidMount(), I call focus() on the DOM node directly via the ref. This way, I gain control over the DOM manipulation. Refs are often used in cases where you need fine-grained control over DOM elements outside React’s rendering process.

Explore: How Can You Pass Props to Children Components in React?

2. Why should we not update the state directly?

In React, I should not directly modify the state because React components rely on immutable state updates to properly track changes and re-render components as needed. When the state is updated directly, the component may not re-render because React will not detect the change. Instead of modifying the state directly, React provides the setState() method to handle updates, which ensures the component is properly re-rendered after the state has been modified.

Moreover, directly changing the state can lead to unexpected behavior, especially when multiple state updates are batched or when asynchronous code is involved. React batches updates for performance reasons, so if I update the state directly, I might miss out on React’s optimized rendering process. To ensure consistency and proper updates, it’s crucial to always use setState(), as it takes care of merging the updated state with the current state and triggers a re-render when necessary.

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3. What is the use of refs?

Refs in React provide a way to access and interact with DOM elements directly, without having to go through the state or props system. I typically use refs when I need to imperatively control or manipulate DOM elements, such as focusing an input field, selecting text, or playing/pausing media elements. Refs allow me to bypass React’s usual declarative approach, enabling direct access to underlying DOM nodes.

For instance, in situations where I want to set focus on an input field when the component mounts, I can use a ref:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.inputRef = React.createRef();
  }

  componentDidMount() {
    this.inputRef.current.focus(); // directly accessing the DOM element
  }

  render() {
    return <input ref={this.inputRef} type="text" />;
  }
}

In this example, I use React.createRef() to create a reference to the input field, and then I access the DOM node directly using this.inputRef.current. This kind of control is especially useful when I need to integrate with third-party libraries that manipulate the DOM directly or when I need to optimize certain interactions outside of React’s normal rendering flow.

Another useful application of refs is in managing animations or performing complex DOM operations that React’s declarative model doesn’t support as easily. However, while refs are powerful, they should be used sparingly, as over-reliance on refs can lead to less maintainable and harder-to-debug code.

See also: Data Science Interview Questions

4. What are forward refs?

Forward refs in React allow me to pass a ref through a component to one of its child DOM elements. This is particularly useful when I have a higher-order component or a wrapper component, and I still want to provide the parent component with access to a specific DOM node within the child component. Normally, refs are only accessible to the component where they are created, but by using React.forwardRef(), I can forward a ref from a parent component to a specific DOM element in a child component.

Here’s a simple example of forward refs in action:

const FancyButton = React.forwardRef((props, ref) => (
  <button ref={ref} className="fancy-button">
    {props.children}
  </button>
));

const App = () => {
  const buttonRef = React.createRef();

  return (
    <FancyButton ref={buttonRef}>
      Click me
    </FancyButton>
  );
};

In this example, the FancyButton component is a child component that uses React.forwardRef() to allow the parent (App component) to pass a ref directly to the button element. By doing so, I can manipulate the button element, like focusing it or triggering other events directly from the parent component, which otherwise would not be possible without exposing internal implementation details.

Forward refs are especially useful when building component libraries, where the internal structure of a component is hidden but you still need to expose access to certain DOM elements for external manipulation. It ensures flexibility and keeps the encapsulation intact.

Explore: Event Handling in Reactjs

5. What are the different phases of component lifecycle?

React components have distinct lifecycle phases: mounting, updating, and unmounting. During these phases, certain methods (lifecycle methods) are called automatically to allow developers to hook into the component’s lifecycle. The mounting phase occurs when a component is first created and inserted into the DOM, with methods like constructor(), componentDidMount(), and render() being called.

Next comes the updating phase, which happens when the component’s state or props change. During this phase, methods such as componentDidUpdate() and shouldComponentUpdate() are triggered to manage the re-rendering of the component. Finally, the unmounting phase happens when the component is about to be removed from the DOM. In this phase, componentWillUnmount() is called, allowing me to clean up any resources, like event listeners or timers, before the component is destroyed.

Explore: Form Handling in React JS

6. What will happen if you use setState in constructor?

Using setState() in the constructor of a React component is not recommended and will likely lead to errors. The constructor is used primarily for initializing state and binding methods in class components, but at the point the constructor is called, the component is not fully initialized, and the React lifecycle has not yet started. When I call setState() inside the constructor, it attempts to update the state before React has fully prepared the component, which can result in unintended behavior.

The proper place to use setState() is after the component has mounted, typically within the componentDidMount() lifecycle method. This is because, by that time, the component is ready, and any state changes will trigger a re-render. In the constructor, I can directly set the initial state using this.state = {...} instead of setState().

Here’s an example:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 }; // directly set initial state here
  }
  
  componentDidMount() {
    this.setState({ count: this.state.count + 1 }); // safe to use setState here
  }
  
  render() {
    return <div>{this.state.count}</div>;
  }
}

In the example, I initialize count in the constructor using this.state, and later safely update it using setState() in componentDidMount(). This approach ensures React properly handles the state and triggers the necessary re-renders.

See also: Infosys AngularJS Interview Questions

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

In React, there are several ways I can pass parameters to event handlers or callbacks. One common method is using an arrow function inside the event handler, where I explicitly pass arguments to the function. Arrow functions are concise and allow me to pass both the event object and additional parameters to the handler

For example:

 class MyComponent extends React.Component {
  handleClick = (param, event) => {
    console.log(param); // logs the custom parameter
    console.log(event); // logs the event object
  };

  render() {
    return (
      <button onClick={(event) => this.handleClick('clicked', event)}>
        Click me
      </button>
    );
  }
}

In this example, I pass both 'clicked' as a custom parameter and the event object to the handleClick method using an arrow function in the onClick handler. This technique is simple and commonly used.

Another approach is to use the bind() method to create a bound function with the desired parameters. The bind() method allows me to predefine the arguments that will be passed to the event handler when the event occurs:

<button onClick={this.handleClick.bind(this, 'clicked')}>
  Click me
</button>

With bind(), I pass 'clicked' as the first argument to the handleClick method, and when the event occurs, the bound function is invoked with this pre-set argument. Both arrow functions and bind() are widely used methods, depending on the situation and preference.

Explore: React Router Interview Questions

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

The callback function in setState() serves a very important purpose when I need to perform an action after the state has been updated. Since setState is asynchronous in nature, relying solely on the next line of code might not give the updated state immediately. To handle scenarios where I need to perform logic that depends on the updated state, I can pass a callback function as the second argument to setState(). This ensures that the callback is executed only after the state update has been completed and the component has re-rendered.

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For instance, in scenarios where I want to perform some task like logging the new state or triggering another action based on the state change, the callback is ideal:

this.setState({ count: this.state.count + 1 }, () => {
  console.log('State updated:', this.state.count);
});

Here, after updating the count in the state, the callback logs the updated count. Without the callback, the console.log might print the previous state value due to the asynchronous nature of state updates. This approach gives me control over executing certain logic at the right time, ensuring the state has been properly updated.

In practice, I often use this feature when there’s a need to coordinate actions after a state change, especially in cases where I’m dealing with complex side effects or need to integrate the state change with another system.

Explore: React Hooks: Revolutionizing Functional Components

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

Binding methods or event handlers in JSX callbacks is necessary when I want to ensure that this in the method refers to the component instance, and not the event context. In React class components, methods like event handlers don’t have access to the correct this context by default, so I need to bind them manually. One common way I do this is by using the bind() method in the constructor of the component.

For example, I can bind a method in the constructor like this:

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

  handleClick() {
    console.log(this); // refers to component instance
  }

  render() {
    return <button onClick={this.handleClick}>Click me</button>;
  }
}

In this code, the bind() method ensures that handleClick has the correct this reference when invoked. This is important because, without binding, this would be undefined or refer to the global object in strict mode. The constructor binding pattern is widely used in class components and ensures that I don’t create a new function on every render, which can have performance benefits.

An alternative approach is using arrow functions, which automatically bind this to the component instance, without the need for bind() in the constructor. Arrow functions are concise and make code more readable:

class MyComponent extends React.Component {
  handleClick = () => {
    console.log(this); // automatically bound
  };

  render() {
    return <button onClick={this.handleClick}>Click me</button>;
  }
}

With arrow functions, the need to explicitly bind methods in the constructor is eliminated. Arrow functions are often preferred in modern React class components for their simplicity and ease of use. However, it’s important to keep in mind that arrow functions created in the render method can have performance implications if not used carefully.

Explore: Lifecycle Methods in React

10.Explain and Implement a Custom Hook in React

A custom hook is a reusable function in React that encapsulates logic that can be shared across multiple components. It allows you to abstract away stateful logic so that it can be reused. A custom hook is just a JavaScript function whose name starts with “use“, and it can use other hooks like useState and useEffect.

Here’s an example of a custom hook that handles input field state:

import { useState } from "react";

// Custom hook to manage input field state
function useInput(initialValue) {
  const [value, setValue] = useState(initialValue);

  const handleChange = (e) => {
    setValue(e.target.value);
  };

  const reset = () => {
    setValue(initialValue);
  };

  return [value, handleChange, reset];
}

// Usage of the custom hook in a component
function InputComponent() {
  const [text, handleChange, reset] = useInput("");

  const handleSubmit = () => {
    alert(`Submitted: ${text}`);
    reset();
  };

  return (
    <div>
      <input type="text" value={text} onChange={handleChange} />
      <button onClick={handleSubmit}>Submit</button>
    </div>
  );
}

export default InputComponent;

In this example, we create a custom hook called useInput to manage the state of an input field. The hook uses useState to store the input value and provides functions to handle changes and reset the value. This custom hook is then utilized in the InputComponent, allowing for clean and reusable input handling logic. By encapsulating this functionality, we simplify state management across different components.

11.Explain React’s useEffect Hook and Its Dependency Array

useEffect is a hook that allows you to perform side effects in functional components. These effects could be data fetching, subscriptions, or manually changing the DOM. useEffect takes two arguments: a callback function and an optional dependency array. The callback function runs when the component mounts, and the dependency array determines when the effect should re-run.

Here’s an example of fetching data from an API when a component mounts and re-fetching it only when a userId prop changes:

import React, { useState, useEffect } from "react";

function UserProfile({ userId }) {
  const [userData, setUserData] = useState(null);

  // useEffect to fetch user data on mount and when userId changes
  useEffect(() => {
    const fetchUserData = async () => {
      const response = await fetch(`https://jsonplaceholder.typicode.com/users/${userId}`);
      const data = await response.json();
      setUserData(data);
    };

    fetchUserData();
  }, [userId]); // Effect depends on userId

  if (!userData) return <div>Loading...</div>;

  return (
    <div>
      <h2>{userData.name}</h2>
      <p>{userData.email}</p>
    </div>
  );
}

export default UserProfile;

This example, demonstrates the use of the useEffect hook to fetch user data from an API when the component mounts and when the userId prop changes. The hook contains a callback function that asynchronously retrieves data and updates the component’s state. The dependency array [userId] ensures that the effect only runs when the userId changes, preventing unnecessary API calls. This showcases how useEffect effectively manages side effects in functional components.

Explore: Component Composition in React

12. Which is preferred option within callback refs and findDOMNode()?

Between callback refs and findDOMNode(), the preferred option is callback refs. Callback refs are a more declarative and safe way to handle DOM manipulation in React. With callback refs, I can directly assign a function to the ref attribute, which gives me full control over the lifecycle of the DOM node. This approach is both flexible and aligns well with React’s component model.

For example, a callback ref can be implemented like this:

class MyComponent extends React.Component {
  setRef = (element) => {
    this.myRef = element;
    if (this.myRef) {
      this.myRef.focus(); // directly interact with the DOM element
    }
  };

  render() {
    return <input ref={this.setRef} type="text" />;
  }
}

In the above example, the setRef function gets called with the DOM element when the component mounts, and I can manipulate the DOM directly. Callback refs provide a cleaner and safer approach, especially when compared to findDOMNode().

On the other hand, findDOMNode() is considered legacy and unsafe in newer versions of React. It was typically used in class components to access the DOM element of a child component, but it comes with performance issues and potential conflicts with React’s virtual DOM. Moreover, findDOMNode() does not work in strict mode and is not compatible with concurrent rendering in React. This is why callback refs are generally recommended over findDOMNode() in modern React development.

Explore: Conditional Rendering in React

13. Why are String Refs legacy?

String refs were a method used in earlier versions of React to assign refs by using string values, but they have since been deprecated and are considered legacy. The reason string refs are discouraged is that they are inefficient and less safe compared to modern ref techniques like callback refs and React.createRef(). When I use string refs, React internally handles the ref differently, which introduces unnecessary complexity and can lead to performance issues.

For instance, here’s how string refs were used in older React versions:

class MyComponent extends React.Component {
  render() {
    return <input ref="myInput" type="text" />;
  }

  componentDidMount() {
    this.refs.myInput.focus(); // Accessing the ref via string
  }
}

This method relied on the this.refs object to access the DOM element, but string refs had issues with asynchronous updates and can lead to bugs, especially in more complex component hierarchies.

The modern way to handle refs is through callback refs or using React.createRef(), which are more consistent and predictable. These methods ensure that the ref is properly managed throughout the component’s lifecycle and provide better integration with React’s concurrent rendering features. For these reasons, string refs are now considered an anti-pattern in modern React development and should be avoided in favor of more reliable alternatives.

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

In React class components, when extending a class (such as React.Component), I must use the super() function to call the parent class’s constructor. When I pass props to the constructor, it ensures that props are properly initialized and available within the class. The reason for this is that this.props is undefined until the parent constructor is called. By passing props to super(), I ensure that this.props is correctly set up and can be accessed within the component.

For example, here’s a typical class component setup:

class MyComponent extends React.Component {
  constructor(props) {
    super(props); // calling the parent constructor and passing props
    this.state = { count: 0 };
  }

  render() {
    return <div>{this.props.name}</div>;
  }
}

In this example, props are passed to the super() constructor, allowing me to access this.props.name in the render method. If I didn’t pass props to super(), the component would not have access to this.props, and trying to use it would result in an error.

Additionally, calling super(props) ensures that React’s internal mechanisms for handling props are properly set up. This practice is required in class components, and omitting super(props) would break the component’s ability to access props, which could lead to unexpected issues during the component’s lifecycle.

Explore: Step-by-Step Guide to React’s Context API

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

In React, I can set state with a dynamic key by using the computed property name syntax in JavaScript. This technique allows me to update a specific state property dynamically, depending on the situation. When I want to update an object property in the state and the key is determined at runtime, I can use square brackets [] to set the key based on a variable. This is particularly useful in forms, where the state is updated based on input field names.

Here’s an example of dynamically updating state:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      name: '',
      age: 0
    };
  }

  handleChange = (event) => {
    const { name, value } = event.target;
    this.setState({
      [name]: value // dynamically setting the key based on the input name
    });
  };

  render() {
    return (
      <div>
        <input name="name" onChange={this.handleChange} />
        <input name="age" onChange={this.handleChange} />
      </div>
    );
  }
}

In this example, the handleChange() method dynamically updates the state using the input field’s name as the key. If I type in the “name” input field, the name state gets updated, and if I type in the “age” input field, the age state gets updated. This dynamic state update method is efficient when managing forms with multiple inputs, where each input can update a different part of the state.

By using this approach, I ensure that the code remains flexible and can handle a variety of inputs without writing separate handlers for each field, keeping the component concise and maintainable.

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

One common mistake that I may encounter is defining a function directly inside the render method. When I do this, the function gets recreated every time the component re-renders, which can lead to performance issues, especially in larger applications. In React, each time a component re-renders, the render method is executed, and if a function is defined within it, that function is recreated with each render, which may also trigger unnecessary re-renders of child components.

For example:

class MyComponent extends React.Component {
  render() {
    return <button onClick={() => console.log('clicked')}>Click Me</button>;
  }
}

In this case, the arrow function inside onClick is re-created on every render. While this might seem harmless in smaller components, it becomes problematic in more complex components with many child elements, where each render could potentially cause the children to re-render unnecessarily.

A better approach is to define the function as a class method or use an arrow function outside the render method to avoid recreating the function on each render:

class MyComponent extends React.Component {
  handleClick = () => {
    console.log('clicked');
  };

  render() {
    return <button onClick={this.handleClick}>Click Me</button>;
  }
}

In this example, the handleClick function is defined outside of the render method, so it’s not recreated with every re-render. This approach ensures that the function remains the same between renders, preventing unnecessary child re-renders and improving the overall performance of the component.

This is especially important when passing functions down as props to child components, as React performs shallow comparison of props, and a newly created function reference would cause the child to re-render even if nothing else has changed.

Read more : Conditional Rendering in React JS

17. What are error boundaries in React v16?

Error boundaries in React v16 are special components that catch JavaScript errors anywhere in their child component tree, log the error, and display a fallback UI instead of crashing the whole application. This feature is extremely useful because, prior to v16, React components would break the entire UI if an error occurred during rendering, making error handling challenging. With error boundaries, I can ensure that an error in one part of the UI doesn’t affect the rest of the application.

Error boundaries work by implementing two lifecycle methods: componentDidCatch(error, info) and getDerivedStateFromError(). The first method logs the error and provides additional information about it, while the second is used to render a fallback UI when an error is detected.

Here’s a simple example:

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    console.log("Caught an error:", error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

In this example, the ErrorBoundary component wraps its children and catches any error that occurs during rendering. If an error is caught, a fallback UI (Something went wrong.) is displayed. This pattern ensures that the rest of the application remains functional, even if a specific component fails.

It’s important to note that error boundaries only catch errors in the rendering phase and lifecycle methods, not in event handlers. For event handlers, I still need to use traditional try-catch blocks to manage errors.

Explore: Understanding React.js Props and State with Practical Examples

18. How are error boundaries handled in React v15?

In React v15 and earlier versions, there was no official concept of error boundaries. When a JavaScript error occurred within the render method of a component or a lifecycle method, it would cause the entire component tree to unmount, and React provided no built-in way to handle these errors. This led to situations where the entire UI could crash without a graceful recovery mechanism. If I wanted to handle errors in React v15, I would typically rely on try-catch blocks manually placed around critical parts of the code, especially in event handlers, but this wasn’t always sufficient for handling errors within the render phase.

Developers had to resort to global error handling mechanisms using tools like window.onerror or third-party libraries such as Sentry or Rollbar to capture and report errors. But these methods only provided post-error logging, without the ability to recover the UI or show a fallback component gracefully.

Here’s an example of how error handling was typically done before error boundaries in v15

class MyComponent extends React.Component {
  componentDidMount() {
    try {
      // Risky operation
    } catch (error) {
      console.log("An error occurred:", error);
    }
  }

  render() {
    return <div>My Component</div>;
  }
}

While try-catch blocks work for handling errors in lifecycle methods or event handlers, they do not address the issue of rendering errors. In React v16 and beyond, error boundaries solve this limitation by enabling me to handle rendering errors at the component level, preventing the entire app from crashing.

Explore: Creating a Sample Service in React JS

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

The render() method from the react-dom package is essential in bootstrapping a React application to the DOM. This method takes a React component tree and mounts it onto a specified DOM element, effectively linking the virtual React tree to the real browser DOM. Without this method, I wouldn’t be able to display React components in a web page.

Here’s how I typically use the render() method:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));

In this example, the App component is rendered inside the DOM element with the ID root. The render() method does several things: it reconciles the virtual DOM with the actual DOM, triggers initial component mounting, and begins the React lifecycle for the components in the tree. When state or props change in the components, render() handles the re-rendering of the necessary parts of the UI without having to reload the entire page.

The render() method is a core part of React’s declarative rendering model, where I specify how the UI should look based on the current state, and React efficiently updates the DOM as the state changes. This makes React highly performant compared to traditional imperative DOM manipulation techniques.

Explore: Props and State in React

20. What will happen if you use setState in constructor?

Using setState() inside the constructor of a React class component is not allowed because the component hasn’t been fully initialized yet when the constructor is running. The constructor is where I can directly assign the initial state using this.state = {...}, but React’s internal mechanisms for managing state are not yet available. If I try to call setState() in the constructor, React will throw an error because the component instance isn’t fully prepared to handle state changes at that point.

Instead of calling setState() in the constructor, I should initialize state by assigning it directly. The proper place to use setState() is in the componentDidMount() lifecycle method or other lifecycle methods where the component has been fully mounted and is part of the DOM. Here’s a better approach:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 }; // Initializing state directly in the constructor
  }

  componentDidMount() {
    this.setState({ count: this.state.count + 1 }); // Safe to use setState here
  }

  render() {
    return <div>{this.state.count}</div>;
  }
}

In this example, I initialize the count state in the constructor and update it using setState() in the componentDidMount() method. This approach ensures that React’s state management mechanisms work correctly and avoids any potential errors or undefined behavior.

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