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: In React, constructors are used only in the following two cases: By giving Bind an instance for the event handler. If you don’t initialize In Can only be specified directly in the constructor 3.13.1. 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'));
this.state
assign an object to initialize the interior
state
.
state
or without method binding, you do not need to implement a constructor for the React component.
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);
}
this.state
assign a value. If you need to assign values in other methods, you should use the
this.setState()
replace.