[javascript] How to update parent's state in React?

It seems that we can only pass data from parent to child as react promotes Unidirectional Data Flow, but to make parent update itself when something happens in its "child component", we generally use what is called a "callback function".

We pass the function defined in the parent to the child as "props" and call that function from the child triggering it in the parent component.


class Parent extends React.Component {
  handler = (Value_Passed_From_SubChild) => {
    console.log("Parent got triggered when a grandchild button was clicked");
    console.log("Parent->Child->SubChild");
    console.log(Value_Passed_From_SubChild);
  }
  render() {
    return <Child handler = {this.handler} />
  }
}
class Child extends React.Component {
  render() {
    return <SubChild handler = {this.props.handler}/ >
  }
}
class SubChild extends React.Component { 
  constructor(props){
   super(props);
   this.state = {
      somethingImp : [1,2,3,4]
   }
  }
  render() {
     return <button onClick = {this.props.handler(this.state.somethingImp)}>Clickme<button/>
  }
}
React.render(<Parent />,document.getElementById('app'));

 HTML
 ----
 <div id="app"></div>

In this example we can make data pass from SubChild -> Child -> Parent by passing function to its direct Child.

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