React constructor() method
constructor() : The format of the method is as follows:
constructor(props)
Before the React component is mounted, its constructor is called constructor() .
React.Component : When a subclass implements a constructor, it should be called before other statements super(props) .
When the following example creates a component, React calls the constructor:
Example
classHeaderextendsReact.Component{constructor(props){super(props);this.state={favoritewebsite:"runoob"};}render(){return(<h1>My favorite websites{this.state.favoritewebsite}</h1>);}}ReactDOM.render(<Header/>,document.getElementById('root'));
In React, constructors are used only in the following two cases:
By giving
this.stateassign an object to initialize the interiorstate.Bind an instance for the event handler.
If you don’t initialize state or without method binding, you do not need to implement a constructor for the React component.
In constructor() do not call the setState() method. If your component needs to use the internal state directly in the constructor for this.state initial assignment state :
constructor(props) {
super(props);
// Do not call here this.setState()
this.state = { counter: 0 };
this.handleClick = this.handleClick.bind(this);
}
Can only be specified directly in the constructor this.state assign a value. If you need to assign values in other methods, you should use the this.setState() replace.