Hooks in React-JS

If we want to implement the react-life cycle activities under the function component then react provides hooks. we do not call the hooks it will automatically call during component loading.

if we want to call the functionality during component update then we can call useEffect hooks, React provide multiple hooks method to implement hooks functionality in an application.

Hooks are similar to JavaScript functions, but you need to follow these two rules when using them. Hooks rule ensures that all the stateful logic in a component is visible in its source code. These rules are:

1. Only call Hooks at the top level

Do not call Hooks inside loops, conditions, or nested functions. Hooks should always be used at the top level of the React functions. This rule ensures that Hooks are called in the same order each time a components renders.

2. Only call Hooks from React functions

You cannot call Hooks from regular JavaScript functions. Instead, you can call Hooks from React function components. Hooks can also be called from custom Hooks.

Hooks State

Hook state is the new way of declaring a state in React app. Hook uses useState() functional component for setting and retrieving state. Let us understand Hook's state with the following example.

Now i am implementig useState() to intialize the value

import React, { useState } from 'react';  

  

function App() {  

  // Declare a new state variable, which we'll call "count"  

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

  

  return (  

    <div>  

      <p>You clicked {count} times</p>  

      <button onClick={() => setCount(count + 1)}>  

        Click me  

      </button>  

    </div>  

  );  

}  

export default App;

React Hooks

In the above example, useState is the Hook which needs to call inside a function component to add some local state to it. The useState returns a pair where the first element is the current state value/initial value, and the second one is a function that allows us to update it. After that, we will call this function from an event handler or somewhere else. The useState is similar to this.setState in class. The equivalent code without Hooks looks like as below.

import React, { useState } from 'react';  

  

class CountApp extends React.Component {  

  constructor(props) {  

    super(props);  

    this.state = {  

      count: 0  

    };  

  }  

  render() {  

    return (  

      <div>  

        <p><b>You clicked {this.state.count} times</b></p>  

        <button onClick={() => this.setState({ count: this.state.count + 1 })}>  

          Click me  

        </button>  

      </div>  

    );  

  }  

}  

export default App;

Hooks Effect

The Effect Hook allows us to perform side effects (an action) in the function components. It does not use components lifecycle methods which are available in class components. In other words, Effects Hooks are equivalent to componentDidMount(), componentDidUpdate(), and componentWillUnmount() lifecycle methods.

Side effects have common features which the most web applications need to perform, such as:

Updating the DOM,

Fetching and consuming data from a server API,

Setting up a subscription, etc.

Let us understand Hook Effect with the following example.

import React, { useState } from 'react';  

  

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

  

function CounterExample() {  

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

  

  // Similar to componentDidMount and componentDidUpdate:  

  useEffect(() => {  

    // Update the document title using the browser API  

    document.title = `You clicked ${count} times`;  

  });  

  

  return (  

    <div>  

      <p>You clicked {count} times</p>  

      <button onClick={() => setCount(count + 1)}>  

        Click me  

      </button>  

    </div>  

  );  

}  

export de

export default App;

Custom hooks:-

we can create our own hooks function, it will cal the user defined hooks.

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

const useDocumentTitle = title => {  

  useEffect(() => {  

    document.title = title;  

  }, [title])  

}    

function App() {  

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

  const incrementCount = () => setCount(count + 1); 

  useDocumentTitle(`You clicked ${count} times`);  

  return (  

    <div>  

      <p>You clicked {count} times</p>  

      <button onClick={incrementCount}>  

        Click me  

      </button>  

    </div>  

  );  

}  

export default App;