反应：当新的道具传递给组件时，构造函数中声明的this.var不会更新

Question

I am unable to understand why a variable initialized in constructor is not updated when new props is passed to child component.

when new props is sent to the component, this.data remains as the old prop's values. (Note this is just an example without any state changes to demonstrate, i know that using states is an alternative method)

constructor(props) {
    super(props)
    this.data = this.props.receivedData 
}

  render() {
    console.log(this.props.receivedData) -> prints new props data
    console.log(this.data) -> retains old props data
    return (
      <svg
        ....
      </svg>
    )
  }

Answer 1

The constructor doesn't get called again when new props are passed. It used to be that you would hook into the componentWillReceiveProps lifecycle method. However, that is considered bad juju as of late and you should use the getDerivedStateFromProps method instead.

See the official docs .

This article does a decent job of explaining it.

Answer 2

A constructor is called only first time when the component is created, where you are assigning the value. Next time you will receive the props in ComponenantWillReceiveProps(nextProps) method. Try following code:

 constructor(props) {
   super(props)
   this.data = this.props.receivedData;
}
ComponenantWillReceiveProps(nextProps){
    if(this.props.receivedData !== nextProps.receivedData){
         this.data = nextProps.receivedData;//new data will be updated here
      }
}
render() {
    console.log(this.props.receivedData)
    console.log(this.data)
    return (
      <svg
      </svg>
    )
 }

you can use state to update the value and render the view when new data is received.

Ex:-

constructor(props) {
   super(props)
   this.data = this.props.receivedData;
   this.state = {
       data : this.props.receivedData
   };

}
ComponenantWillReceiveProps(nextProps){
    if(this.props.receivedData !== nextProps.receivedData){
         this.setState({data:nextProps.receivedData});//new data will be updated here
      }
}
render() {
    console.log(this.props.receivedData)
    console.log(this.data)
    return (
        <div>{this.state.data}</div>
    )
 }

Answer 3

Just to expand a little on the other answers. Depending on what you are trying to do, there are different ways of going about this. As the other answers state, the constructor will only run one time on the construction of the component.

Now if you want to take the incoming props and store them as state, and then have that state updated as a result of new props then getDerivedStateFromProps might be what you need. If this is what you want however, its important to know why you want to do this in the first place because you may not need to. One common reason which causes people to opt for this pattern is to memoize props, but this can and should be achieved using a memoizing library. You can learn more about this here.

The other reason why you might opt for this, is to simply have your component update as a result of props changing. Now you hinted to this in your question, but for the sake of clarity, all you would need to do is use the incoming props, and when they change the component would update as a result. In other words, in order to simply render the props, there is no reason to store them in state or instance variable.