What is Constructor?
- The constructor is a method that initializes an state within a class and is automatically called when an object of the class is created.
- The concept of a constructor in React is similar. The constructor in a React component is called before the component is mounted. When implementing the constructor, it's important to call the `super(props)` method before any other statements. Failing to call `super(props)` will result in `this.props` being undefined in the constructor, which can lead to bugs.
Syntax
Constructor(props){
super(props);
}
In React, constructors are primarily used for two purposes:
1. Initializing the component's local state by assigning an object to `this.state`.
2. Binding event handler methods within the component.
Note: If you don't need to initialize state or bind methods in your React component, there's no need to implement a constructor.
You cannot call the `setState()` method directly within the constructor(). If the component requires local state, you should assign the initial state directly to `this.state` in the constructor. The constructor should only be used to set the initial state, while all other state updates should be made using the `setState()` method.
Example:
The concept of the constructor can be understood through the following example.
App.js
import React, { Component } from 'react';
class MyComponent extends Component {
constructor(props){
super(props);
this.state = {
name: 'James',
age: 20
};
this.handleClick = this.handleClick.bind(this);
}
handleClick(){
console.log('Name:', this.state.name);
console.log('Age:', this.state.age);
}
render() {
return (
<div className="MyComponent">
<h2>React Constructor Example</h2>
<input type="text" value={this.state.name} readOnly />
<p>Age: {this.state.age}</p>
<button onClick={this.handleClick}>Show Details</button>
</div>
);
}
}
export default MyComponent;
Main.js
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App.js';
ReactDOM.render(<App />, document.getElementById('app'));
Output:
When you execute the above code, you get the following output.
The most common question related to the constructor are:
1.Is it required to have a constructor in every component?
No, a constructor is not required in every component. If the component is simple, it just returns a node.
class App extends Component {
render () {
return (
<p> Name: { this.props.name }</p>
);
}
}
2. Is it required to call super() inside a constructor?
Yes, calling `super()` inside a constructor is necessary. If you need to set a property or access `this` within the constructor of your component, you must call `super()` first.
class App extends Component {
constructor(props){
this.fName = "James"; // 'this' is not allowed before super()
}
render () {
return (
<p> Name: { this.props.name }</p>
);
}
}
When you run the code, you'll encounter an error saying that `'this' is not allowed before super()`. Therefore, if you need to access `props` inside the constructor, you must call `super(props)` first.
Arrow Functions:
Arrow functions are a feature introduced in the ES6 standard. When using arrow functions, there is no need to bind `this` to any event because the scope of `this` is lexically bound, meaning it is not limited to the calling function. Therefore, if you're using arrow functions, you don't need to bind `this` inside the constructor.
import React, { useState } from 'react';
const App = (props) => {
// Using the useState hook to manage state
const [data, setData] = useState('www.gocourse.com');
// Handle the button click event
const handleEvent = () => {
console.log(props); // Accessing props here
};
return (
<div className="App">
<h2>React Constructor Example with Functional Component</h2>
<input type="text" value={data} readOnly />
<button onClick={handleEvent}>Please Click</button>
</div>
);
};
export default App;
We can use a constructor in the following ways:
1.The constructor is used to set up the initial state.
class App extends Component {
constructor(props){
// here, it is setting initial value for 'inputTextValue'
this.state = {
inputTextValue: 'initial value',
};
}
}
2.Using `this` inside the constructor allows you to access the component's properties and methods.
class App extends Component {
constructor(props) {
// when you use 'this' in constructor, super() needs to be called first
super();
// it means, when you want to use 'this.props' in constructor, call it as below
super(props);
}
}
3.Integrating third-party libraries.
class App extends Component {
constructor(props) {
this.myBook = new MyBookLibrary();
//Here, you can access props without using 'this'
this.Book2 = new MyBookLibrary(props.environment);
}
}
4.Binding the context (`this`) is necessary when you need to pass a class method as a prop to a child component.
class App extends Component {
constructor(props) {
// when you need to 'bind' context to a function
this.handleFunction = this.handleFunction.bind(this);
}
}
