Overview

React Hooks, introduced in React 16.8, allow you to use state and other React features without writing a class. Understanding useState, useEffect, and useContext is crucial for managing state, side effects, and accessing React context in functional components, respectively. They simplify the code, make it more readable, and help in managing the lifecycle and context in a more straightforward way.

Key Concepts

• State Management with useState: Managing local state in a functional component.
• Side Effects with useEffect: Executing side effects in functional components.
• Context API with useContext: Accessing React context without using a Consumer component.

Common Interview Questions

Basic Level

1. What are React Hooks?
2. How do you use the useState hook in a functional component?

Intermediate Level

3. How does the useEffect hook work for managing side effects?

Advanced Level

4. Explain how you would optimize a component that uses useContext to avoid unnecessary re-renders.

Detailed Answers

1. What are React Hooks?

Answer: React Hooks are functions that let you “hook into” React state and lifecycle features from functional components. They provide a more direct API to the React concepts you already know: props, state, context, refs, and lifecycle. Hooks do not work inside classes — they let you use React without classes.

Key Points:
- Hooks allow the use of state and other React features without writing a class.
- They enable better code organization and reuse without the complexity of higher-order components or render props.
- Hooks were introduced to simplify component logic and make it easier to adopt React.

Example:

// Unfortunately, the request was to use C# for examples, which is not applicable to React. React code is written in JavaScript or TypeScript.
// Here's how the example would look in JavaScript for a simple useState hook usage:

function ExampleComponent() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

2. How do you use the useState hook in a functional component?

Answer: useState is a Hook that allows you to have state variables in functional components. You pass the initial state to this function and it returns a variable with the current state value (not necessarily the initial state) and another function to update this value.

Key Points:
- useState gives you the current state and a function to update it.
- It replaces this.setState in class components.
- It can be used multiple times in a single component to track different state variables.

Example:

// Again, demonstrating with pseudo-JavaScript, as C# does not apply to React:

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>The count is {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

3. How does the useEffect hook work for managing side effects?

Answer: useEffect lets you perform side effects in functional components. It's similar to componentDidMount, componentDidUpdate, and componentWillUnmount in class components. By using this Hook, you tell React that your component needs to do something after render. React will remember the function you passed (we’ll refer to it as our “effect”), and call it later after performing the DOM updates.

Key Points:
- useEffect runs after every render by default.
- It replaces several lifecycle methods by combining them into a single API.
- You can control when an effect runs by passing an array of values as the second argument.

Example:

// JavaScript example for useEffect:

function Example() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

4. Explain how you would optimize a component that uses useContext to avoid unnecessary re-renders.

Answer: When a context changes, all consuming components re-render. To optimize, you can split your context into multiple contexts based on which parts of the state change frequently. Also, memoizing the context provider value with useMemo prevents unnecessary renders by ensuring that the provider value does not change between renders unless its content changes.

Key Points:
- Splitting context into multiple contexts based on update frequency can reduce unnecessary re-renders.
- Memoizing the provider value with useMemo prevents unnecessary context consumers from re-rendering.
- Using React.memo for child components that consume the context can further prevent unnecessary renders.

Example:

// JavaScript example for useContext optimization:

const AppContext = React.createContext();

function AppProvider({ children }) {
  const [user, setUser] = useState(null);
  const value = useMemo(() => ({ user, setUser }), [user]);

  return <AppContext.Provider value={value}>{children}</AppContext.Provider>;
}

function useAppContext() {
  return useContext(AppContext);
}

Again, please note that the code examples are provided in JavaScript, as React is a JavaScript library and C# cannot be used to demonstrate React Hooks.