passing data from child to parent component - react - via callback function

There are a couple of issues.

1) You have to bind passingProps

constructor(){
    super();
    this.state={
      text:''
    }
    this.passingProps = this.passingProps.bind(this);
}

2) this.setState is asynchronous, so it's not guaranteed that this.state.text will be set to the value you want by the time you pass it to this.props.getInput. You can either do

this.props.getInput(newInput)

or

this.setState({ text: newInput }, () => {
  this.props.getInput(this.state.text);
})

to resolve that issue.

class App extends React.Component{
constructor(props){
  super(props);
  this.state={
    input:'this is the input for now'
  }
  this.handleInput=this.handleInput.bind(this);   
}

handleInput(event){
  let value = event.target.value;
  this.setState({
    input:value
  });
}

render(){
   return(
     <div>
       <h1>{this.state.input}</h1>
       <Child getInput={this.handleInput} />
     </div>
   );
  }
}

 class Child extends React.Component{
   constructor(){
      super(props);
}

render(){
   return(
     <div>
     <input type="text" placeholder="please input a name..." onChange={this.props.getInput} />
    </div>

     )
   }
}

ReactDOM.render(
  <App/>,
  document.getElementById('app')
);

Here is the answer for your question. I hope your proplem is solved.