Most frequently asked React Hooks Interview Questions

1. What are React Hooks?
2. What Are The Advantages of react hooks?
3. What are the benefits of using Custom Hooks?
4. Can you give an example of a simple Custom React Hook?
5. Do two components that use the same Hook have their own separate state?
6. How is useState different from useRef in React Hooks?
7. How do React Hooks help a developer write code?
8. How do React Hooks help a developer write code?
9. What role does componentWillMount() play in React Hooks?
10. What is the purpose of the useCallback hook?
11. What is the purpose of the useMemo hook?
12. What is the purpose of useEffect in React Hooks?

What are React Hooks?

React Hooks are functions that allow developers to use features such as state and lifecycle methods in functional components. They make it possible for a function component to have similar capabilities to class components, without the need for writing class components. Hooks can also be used to share logic and data between different components. Essentially, React Hooks allow functional components to act like class components and manipulate variables, call functions from external sources, and generally control the flow of the application. Hooks are easy to use and help developers to keep the codebase uncluttered; the same logic can be shared among the components without making them overly complex.

What Are The Advantages of react hooks?

React Hooks offer a range of advantages over traditional class-based components, such as improved code readability and reusability. Firstly, they allow developers to use state and lifecycle methods in functional components, eliminating the need to switch between class and function components. Secondly, they enable sharing of logic and data between components, making it easier to manage the codebase. Thirdly, through the use of hooks, the application's flow is more easily controlled, allowing for the creation of complex yet unified applications. Finally, hooks keep the codebase clean by enabling the same logic to be utilized for different components without making them overly complicated. Ultimately, React Hooks make it possible for developers to write powerful and scalable applications with fewer lines of code, making for an efficient and enjoyable development experience.

What are the benefits of using Custom Hooks?

Custom Hooks are beneficial because they can be used to write functions that are reusable across different components in React. They provide a way to hook into React state and lifecycle behaviour from within a function component. Hooks allow developers to build complex applications with small, contained, decoupled code. With Custom Hooks, it is easier to share logic among components and break down logic into smaller pieces more easily. This makes the code more maintainable and easier to read for other developers. It also helps in developing programs that are more flexible, as parameters and options can be passed through the Custom Hook to meet your specific needs for each situation, thereby reducing the amount of code you need to write. Custom Hooks enable developers to use all the features of React without needing to create class components. By using Custom Hooks, you can take advantage of the built-in hooks provided by React, such as useState and useEffect. This helps developers to focus on the code they need to write, and helps them achieve better performance in the long term. Furthermore, Custom Hooks are a great way to enforce good software design principles such as DRY (Don't Repeat Yourself). By breaking down code into smaller functions, it becomes easier to test, debug, and refactor your code. This ensures a high level of maintainability and reliability across your application. In short, Custom Hooks are an easy way to add more flexibility and maintainability to your React applications, while still taking advantage of the features provided by React.

Can you give an example of a simple Custom React Hook?

Sure, here is an example of a simple Custom React Hook that takes in two parameters: a counter and a limit. The hook will increment the counter by one every time it's called, and limit how many times it can be called before it reaches the given limit.

// custom hook example 
const useCounterHook = (counter, limit) => {
  const [count, setCount] = useState(counter);

  // increment counter every time hook is invoked
  const increment = () => setCount(count + 1);

  // if counter is greater than or equal to limit return
  // the number of times hook has been invoked
  const getTimesInvoked = () => (count >= limit ? count : null);

  return { increment, getTimesInvoked };
};

// example usage in component
const ExampleComponent = () => {
  const { increment, getTimesInvoked } = useCounterHook(0, 3);

  // call hook 3 times before stopping
  useEffect(() => {
    if (!getTimesInvoked()) {
      increment();
    }
  }, [increment, getTimesInvoked]);

  return <div>Example Component</div>;
};
</div>

Do two components that use the same Hook have their own separate state?

Yes, two components that use the same Hook will have their own separate state. This is because each component can receive its own individualized state from the Hook, rather than a centralized shared state. The code snippet below shows how two components can use the same Hook but still maintain their own individualized states.

const someHook = () => {
  const [state1, setState1] = React.useState(0);
  const [state2, setState2] = React.useState(0);
 
  return [state1, setState1, state2, setState2];
}; 

const ComponentA = () => {
  const [state1, setState1, , ] = useHook();

  return (
    <div>
      <p>Component A state1 value: {state1}</p>
      <button onClick={() => setState1(state1 + 1)}>Increase state1</button>
    </div>
  )
};

const ComponentB = () => {
  const [, , state2, setState2] = useHook();

  return (
    <div>
      <p>Component B state2 value: {state2}</p>
      <button onClick={() => setState2(state2 + 1)}>Increase state2</button>
    </div>
  )
};

How is useState different from useRef in React Hooks?

useState and useRef are two React Hooks that can be used to set state in a functional component. The primary difference between these two is that useState is used to manage state values that are changed frequently and are tied to components, while useRef is used to store values that do not change often, such as references. The code snippet below shows an example of how useState and useRef can be used in the same component:

const Component = () => {
  const [count, setCount] = React.useState(0);
  const countRef = React.useRef(0);

  React.useEffect(() => {
    document.title = `You have clicked  times`;
  }, [count]);

  return (
    <div>
      <p>Count: {count} setCount(count + 1)}>Increase Count</button>
      <button onClick={() => countRef.current++}>Increase Reference Count</button>
    </div>
  )
};

As seen above, when the "Increase Count" button is pressed, the component will update its count state using the useState hook, while when the "Increase Reference Count" button is pressed, the component will update its count reference using the useRef hook.

How do React Hooks help a developer write code?

React Hooks are a great way for developers to write cleaner and more concise code. With hooks, developers can use features such as state and effects without having to create classes or use class-based components. This means they can avoid the "wrapper hell" sometimes caused by excessive nesting of components and instead use simple, straightforward functions that can quickly be used to add features to an app. For example, instead of writing a class component with logic for setting state, a developer could take advantage of useState hook to do the same thing in fewer lines of code:

const [count, setCount] = React.useState(0);
 
const handleClick = () => {
  setCount(count + 1);
}
 
return (
  <div>
    <p>You have clicked {count} times</p>
    <button onClick={handleClick}>Increase Count</button>
  </div>
);

How do React Hooks help a developer write code?

React Hooks help developers write clean and maintainable code by allowing them to use functions instead of classes for state management. With React Hooks, developers can write code in a functional and declarative style, where the logic and data are all contained within individual functions. This makes it easier to debug and reason about the code, and helps keep the project codebase organized. Using the useState hook, for example, developers can manage their state without having to use a class-based component. Here's an example of how to use useState: const [myValue, setMyValue] = useState("") With this code, we are using useState to declare a state variable called myValue and create a function called setMyValue that will be used to update the value of myValue. When myValue is updated, the component will automatically re-render, making it easier to keep track of the state updates. In summary, React Hooks make writing code simpler, more organized, and less error prone. By allowing developers to write code in a functional style, React Hooks let developers focus on the logic and data, not the structure and complexity of classes.

What role does componentWillMount() play in React Hooks?

In React Hooks, componentWillMount() has been replaced by the useEffect() hook. The useEffect() hook allows developers to execute some logic when a component has been mounted. This helps to prevent unnecessary re-rendering of components which can cause performance issues. Here's an example of how to use useEffect():

useEffect(() => {
 //call API
}, []);

In this example, the useEffect() hook will only be called once when the component is first mounted. This prevents unnecessary API calls and helps with performance. The useEffect() hook provides more flexibility than componentWillMount() as it can be used to execute any kind of logic, not just API calls. Additionally, the useEffect() hook can be used to monitor the changes in state and props or execute cleanup logic, whereas componentWillMount() is limited to only performing one-time logic when the component is mounted. To summarize, the useEffect() hook has replaced the componentWillMount() method in React Hooks and provides greater flexibility for developers. It allows them to execute any kind of logic when a component is mounted, including monitoring state and props changes, executing clean up logic, and more.

What is the purpose of the useCallback hook?

The useCallback hook is a React hook that allows you to create a memoized callback. A memoized callback is a type of function that takes an argument and creates a cached version of the result of the first call to the function. This cached version can then be re-used for subsequent function calls with the same argument. This is useful in many situations where you want to avoid extra computation, such as rendering components or re-running expensive calculations. By using the useCallback hook, you can store the cached results and make sure that your components aren't unnecessarily re-rendered or re-calculated. Here is an example of how to use the useCallback hook:

const MyComponent = () => {
  const [value, setValue] = React.useState(0);
  
  const memoizedCallback = React.useCallback(
    newValue => {
      setValue(newValue);
    },
    [setValue]
  );

  // ... other component code here

  return (
    <MyOtherComponent
      onChange={memoizedCallback}
    />
  )
};

In this example, the useCallback hook is used to create a memoized callback called memoizedCallback. This memoized callback is passed to MyOtherComponent as the onChange handler. This ensures that the function is only created once for the component and that it will not be re-created every time the component is rendered. This helps to ensure that the component's performance is optimized and that unnecessary processing is avoided.

What is the purpose of the useMemo hook?

The useMemo hook is a React hook that allows you to create a memoized version of a value or function. A memoized version of a value is a cached version of the result of the original calculation, which can then be used for subsequent calculations with the same parameters. This is useful in many situations where you want to avoid extra computation, such as rendering components or running expensive calculations. By using the useMemo hook, you can store the cached results and make sure that your components aren't unnecessarily re-rendered or re-calculated. This can help improve your application's performance significantly, especially when dealing with large datasets or deeply nested data. Here is an example of how to use the useMemo hook:

const MyComponent = () => {
  const [value, setValue] = React.useState(0);
  
  const expensiveValueFn = React.useMemo(() => {
    // ... do expensive calculation
    return value + 1;
  }, [value]);

  // ... other component code here

  return (
    <MyOtherComponent
      value={expensiveValueFn}
    />
  )
};

In this example, the useMemo hook is used to create a memoized version of the expensiveValueFn function. This ensures that the function is only called once for the component and that it will not be re-calculated every time the component is rendered. This helps to ensure that the component's performance is optimized and that unnecessary processing is avoided.

What is the purpose of useEffect in React Hooks?

The useEffect React hook allows you to perform side effects such as data fetching, manually changing the DOM, and subscribing to events in function components. It is similar to the componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods in class components. By using the useEffect hook, you can ensure that side effects are only run when necessary and that they're not unnecessarily triggered by changes that don't actually need them. This helps to make sure that your application's performance is optimized. Here is an example of how to use the useEffect hook:

const MyComponent = () => {
  const [value, setValue] = React.useState(0);

  React.useEffect(() => {
    // ... run side effects here
    console.log(`value changed to `);
  }, [value]); // only run effect if value changes

  // ... other component code here

  return (
    <MyOtherComponent
      value={value}
      onChange={setValue}
    />
  )
};

In this example, the useEffect hook is used to create a side effect that logs a message to the console whenever the value changes. This ensures that the side effect will only be triggered when necessary, saving unnecessary processing and improving application performance.