通过prop将状态值传递给子组件

Question

i'm trying to pass the value entered by the user from the app component to the passTicket component. I tried invoking props to pass this state data but I keep getting an undefined error when attempting to access it. I'm new to react and it would be great if someone can help me make sense of what i'm getting wrong. This is a sample of what i'm trying to achieve. This is my main component:

class App extends Component {

  constructor(props){
    super(props);
    this.state = {
      ticket:"",
    };
    this.changeTicket = this.changeTicket.bind(this);
    this.handleSubmit = this.handleSubmit.bind(this);
    this.keyPress = this.keyPress.bind(this);
  }

  changeTicket(e){
    this.setState({
      ticket : e.target.value,
    })
  }

  handleSubmit(){
    this.setState({
      updatedTicket: this.state.ticket
    });
  }

  keyPress(e){
    if (e.keyCode ===13){
      this.handleSubmit();
    }
  }

  render() {
    return (
      <div className="App">
        <header className="App-header">
          <input type="text" placeholder="ENTER TICKET NUMBER" value={this.state.ticket} onKeyDown={this.keyPress} onChange={this.changeTicket}/>
        </header>
      </div>
    );
  }
}

and i'd like to be able to store the updatedTicket value in a variable which I can use in my PassTicket component. this is what i've attempted so far but the error it occurs is the following Uncaught TypeError: Cannot read property 'updatedTicket' of undefined

this is what my second component looks like:

class PassTicket extends Component {

  transferredTicket(){
    const myTicket = this.props.state.updatedTicket;
    return myTicket
  }

  render() {
    return (
        <p>{this.transferredTicket()}</p>
    );
  }
}

Answer 1

When passing down a property from a parent to a child component, the property will be stored onto the props by the name it's passed through. For example:

class Parent extends Component {
  state = {
     ticket: '',
  }

  render() {
    return <ChildComponent updatedTicket={this.state.ticket} />
  }
}


class ChildComponent extends Component {
  static propTypes = {
     updatedTicket: PropTypes.string,
  }

  static defaultProps = {
     updatedTicket: '',
  }

  render() {
    return (
      <div>{this.props.updatedTicket}</div>
    );
  }

}

In the example you've given, it doesn't seem like you're passing the state down to the component you're trying to access it in. In addition, it seems like you're trying to access the updatedTicket as a property of a state object, so just beware of how you're accessing your props.

Therefore, in order to access the updatedTicket property on the child component, you'll first need to import the PassTicket component, instantiate it in the parent ( App ) component, and pass the property down:

<PassTicket updateTicket={this.state.ticket} />

You would then be able to access the string in the PassTicket component like so - this.props.updateTicket

Answer 2

So .state in react is a local state that is only visible to the individual component. You can read more about it here: https://reactjs.org/docs/state-and-lifecycle.html

In order to pass your state around, you need to use the props system. So where you instantiate your component, you can pass in the state of the parent. For example:

<PassTicket ticket={this.state.updatedTicket}/>

Then inside your PassTicket render function, you can access the ticket prop:

render() {
  const { ticket } = this.props
  return (
    <div>{ticket}</div>
  )
}