What is React Hooks?
React Hooks cannot be used in class components. They let us write components without class.
React Hooks are the built-in functions that permit developers for using the state and lifecycle methods within React functional components. These are newly added features made available in React 16.8 version. Each lifecycle of a component is having 3 phases which include mount, unmount, and update. Along with that, components have properties and states. Hooks will allow using these methods by developers for improving the reuse of code with higher flexibility navigating the component tree.
Using Hook, all features of React can be used without writing class components. For example, before React version 16.8, it required a class component for managing the state of a component. But now using the useState hook, we can keep the state in a functional component.
Why were Hooks introduced in React?
React hooks were introduced in the 16.8 version of React. Previously, functional components were called stateless components. Only class components were used for state management and lifecycle methods. The need to change a functional component to a class component, whenever state management or lifecycle methods were to be used, led to the development of Hooks.
Lifecycle of Components
Each component in React has a lifecycle which you can monitor and manipulate during its three main phases.
The three phases are: Mounting, Updating, and Unmounting.
1- Mounting
Mounting means putting elements into the DOM.
React has four built-in methods that gets called, in this order, when mounting a component:
- constructor()
- getDerivedStateFromProps()
- render()
- componentDidMount()
The render() method is required and will always be called, the others are optional and will be called if you define them.
Constructor
The constructor() method is called before anything else, when the component is initiated, and it is the natural place to set up the initial state and other initial values.
The constructor() method is called with the props, as arguments, and you should always start by calling the super(props) before anything else, this will initiate the parent's constructor method and allows the component to inherit methods from its parent (React.Component).
The constructor method is called, by React, every time you make a component:
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
render() {
return (
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
);
}
}
ReactDOM.render(<Header />, document.getElementById('root'));
getDerivedStateFromProps
The getDerivedStateFromProps() method is called right before rendering the element(s) in the DOM.
This is the natural place to set the state object based on the initial props.
It takes state as an argument, and returns an object with changes to the state.
The example below starts with the favorite color being "red", but the getDerivedStateFromProps() method updates the favorite color based on the favcol attribute: Click here for below code testing
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
static getDerivedStateFromProps(props, state) {
return {favoritecolor: props.favcol };
}
render() {
return (
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header favcol="yellow"/>);
render
The render() method is required, and is the method that actually outputs the HTML to the DOM.
class Header extends React.Component {
render() {
return (
<h1>This is the content of the Header component</h1>
);
}
}
ReactDOM.render(<Header />, document.getElementById('root'));
componentDidMount
The componentDidMount() method is called after the component is rendered.
This is where you run statements that requires that the component is already placed in the DOM. Click here for below code testing
At first my favorite color is red, but give me a second, and it is yellow instead:
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
componentDidMount() {
setTimeout(() => {
this.setState({favoritecolor: "yellow"})
}, 1000)
}
render() {
return (
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
2- Updating
The next phase in the lifecycle is when a component is updated. A component is updated whenever there is a change in the component's state or props.
React has five built-in methods that gets called, in this order, when a component is updated:
- getDerivedStateFromProps()
- shouldComponentUpdate()
- render()
- getSnapshotBeforeUpdate()
- componentDidUpdate()
The render() method is required and will always be called, the others are optional and will be called if you define them.
getDerivedStateFromProps
This is the first method that is called when a component gets updated. This is still the natural place to set the state object based on the initial props.
The example below has a button that changes the favorite color to blue, but since the getDerivedStateFromProps() method is called, which updates the state with the color from the favcol attribute, the favorite color is still rendered as yellow:
If the component gets updated, the getDerivedStateFromProps() method is called: Test below code
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
static getDerivedStateFromProps(props, state) {
return {favoritecolor: props.favcol };
}
changeColor = () => {
this.setState({favoritecolor: "blue"});
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<button type="button" onClick={this.changeColor}>Change color</button>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header favcol="yellow" />);
/*
This example has a button that changes the favorite color to blue,
but since the getDerivedStateFromProps() method is called,
the favorite color is still rendered as yellow
(because the method updates the state
with the color from the favcol attribute).
*/
shouldComponentUpdate
In the shouldComponentUpdate() method you can return a Boolean value that specifies whether React should continue with the rendering or not.
The default value is true.
The example below shows what happens when the shouldComponentUpdate() method returns false:
Stop the component from rendering at any update: Test below code
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
shouldComponentUpdate() {
return false;
}
changeColor = () => {
this.setState({favoritecolor: "blue"});
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<button type="button" onClick={this.changeColor}>Change color</button>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
Same example as above, but this time the shouldComponentUpdate() method returns true instead: Test below code
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
shouldComponentUpdate() {
return true;
}
changeColor = () => {
this.setState({favoritecolor: "blue"});
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<button type="button" onClick={this.changeColor}>Change color</button>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
render
The render() method is of course called when a component gets updated, it has to re-render the HTML to the DOM, with the new changes.
The example below has a button that changes the favorite color to blue:
Click the button to make a change in the component's state: Test below code
import React from 'react';
import ReactDOM from 'react-dom/client';
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
changeColor = () => {
this.setState({favoritecolor: "blue"});
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<button type="button" onClick={this.changeColor}>Change color</button>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
getSnapshotBeforeUpdate
In the getSnapshotBeforeUpdate() method you have access to the props and state before the update, meaning that even after the update, you can check what the values were before the update.
If the getSnapshotBeforeUpdate() method is present, you should also include the componentDidUpdate() method, otherwise you will get an error.
The example below might seem complicated, but all it does is this:
When the component is mounting it is rendered with the favorite color "red".
When the component has been mounted, a timer changes the state, and after one second, the favorite color becomes "yellow".
This action triggers the update phase, and since this component has a getSnapshotBeforeUpdate() method, this method is executed, and writes a message to the empty DIV1 element.
Then the componentDidUpdate() method is executed and writes a message in the empty DIV2 element:
Use the getSnapshotBeforeUpdate() method to find out what the state object looked like before the update: Test below code
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
componentDidMount() {
setTimeout(() => {
this.setState({favoritecolor: "yellow"})
}, 1000)
}
getSnapshotBeforeUpdate(prevProps, prevState) {
document.getElementById("div1").innerHTML =
"Before the update, the favorite was " + prevState.favoritecolor;
}
componentDidUpdate() {
document.getElementById("div2").innerHTML =
"The updated favorite is " + this.state.favoritecolor;
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<div id="div1"></div>
<div id="div2"></div>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
componentDidUpdate
The componentDidUpdate method is called after the component is updated in the DOM.
The example below might seem complicated, but all it does is this:
When the component is mounting it is rendered with the favorite color "red".
When the component has been mounted, a timer changes the state, and the color becomes "yellow".
This action triggers the update phase, and since this component has a componentDidUpdate method, this method is executed and writes a message in the empty DIV element: test below code
class Header extends React.Component {
constructor(props) {
super(props);
this.state = {favoritecolor: "red"};
}
componentDidMount() {
setTimeout(() => {
this.setState({favoritecolor: "yellow"})
}, 1000)
}
componentDidUpdate() {
document.getElementById("mydiv").innerHTML =
"The updated favorite is " + this.state.favoritecolor;
}
render() {
return (
<div>
<h1>My Favorite Color is {this.state.favoritecolor}</h1>
<div id="mydiv"></div>
</div>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Header />);
3- Unmounting
The next phase in the lifecycle is when a component is removed from the DOM, or unmounting as React likes to call it.
React has only one built-in method that gets called when a component is unmounted:
componentWillUnmount
The componentWillUnmount method is called when the component is about to be removed from the DOM.
Click the button to delete the header: Test below code
import React from 'react';
import ReactDOM from 'react-dom/client';
class Container extends React.Component {
constructor(props) {
super(props);
this.state = {show: true};
}
delHeader = () => {
this.setState({show: false});
}
render() {
let myheader;
if (this.state.show) {
myheader = <Child />;
};
return (
<div>
{myheader}
<button type="button" onClick={this.delHeader}>Delete Header</button>
</div>
);
}
}
class Child extends React.Component {
componentWillUnmount() {
alert("The component named Header is about to be unmounted.");
}
render() {
return (
<h1>Hello World!</h1>
);
}
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Container />);
Explain about types of Hooks in React.
There are two types of Hooks in React. They are:
1. Built-in Hooks: The built-in Hooks are divided into 2 parts as given below:
Basic Hooks:
- useState(): This functional component is used to set and retrieve the state.
- useEffect(): It enables for performing the side effects in the functional components.
- useContext(): It is used for creating common data that is to be accessed by the components hierarchy without having to pass the props down to each level.
Additional Hooks:
- useReducer() : It is used when there is a complex state logic that is having several sub-values or when the upcoming state is dependent on the previous state. It will also enable you to optimization of component performance that will trigger deeper updates as it is permitted to pass the dispatch down instead of callbacks.
- useMemo() : This will be used for recomputing the memoized value when there is a change in one of the dependencies. This optimization will help for avoiding expensive calculations on each render.
- useCallback() : This is useful while passing callbacks into the optimized child components and depends on the equality of reference for the prevention of unneeded renders.
- useImperativeHandle(): It will enable modifying the instance that will be passed with the ref object.
- useDebugValue(): It is used for displaying a label for custom hooks in React DevTools.
- useRef() : It will permit creating a reference to the DOM element directly within the functional component.
- useLayoutEffect(): It is used for the reading layout from the DOM and re-rendering synchronously.
2. Custom Hooks: A custom Hook is basically a function of JavaScript. The Custom Hook working is similar to a regular function. The “use” at the beginning of the Custom Hook Name is required for React to understand that this is a custom Hook and also it will describe that this specific function follows the rules of Hooks. Moreover, developing custom Hooks will enable you for extracting component logic from within reusable functions.
What are the rules that must be followed while using React Hooks? There are 2 rules which must be followed while you code with Hooks:
- React Hooks must be called only at the top level. It is not allowed to call them inside the nested functions, loops, or conditions.
- It is allowed to call the Hooks only from the React Function Components.
Differentiate React Hooks vs Classes.
|
React Hooks
|
Classes
|
|
It is used in functional components of React.
|
It is used in class-based components of React.
|
|
It will not require a declaration of any kind of
constructor.
|
It is necessary to declare the constructor inside
the class component.
|
|
It does not require the use of this keyword in state declaration or
modification.
|
Keyword this will be used in state declaration (this.state) and in modification (this.setState()).
|
|
It is easier to use because of the useState functionality.
|
No specific function is available for helping us
to access the state and its corresponding setState variable.
|
|
React Hooks can be helpful in implementing Redux
and context API.
|
Because of the long setup of state declarations,
class states are generally not preferred.
|
How does the performance of using Hooks will differ in comparison with the classes?
- React Hooks will avoid a lot of overheads such as the instance creation, binding of events, etc., that are present with classes.
- Hooks in React will result in smaller component trees since they will be avoiding the nesting that exists in HOCs (Higher Order Components) and will render props which result in less amount of work to be done by React.
