无法在componentWillMount中找到属性集

Question

I know I'm missing something basic here and if someone could help I would really appreciate it. This code gives me a warning: "Cannot read property 'setState' of null."

constructor(props) {
    super(props);
    this.state = {
      foundUser: false
}
   }

componentWillMount() {
  var requestEmail = this.props.email
  new Firebase(url)
    .orderByChild('email')
    .startAt(requestEmail)
    .endAt(requestEmail)
    .once('value', function (snap) {
      var foundUser = snap.val();
      if (foundUser) {
        this.setState({found: true})
      } else {
        this.setState({foundUser: false})
      } 
    });
},

render: function () {

  if (this.state.foundUser === false) {
    //render a scene here ...
  }

  return ( ....

Answer 1

You're missing the initial state of the component, try to add following function to your component

getInitialState: function() {
  return {
     foundUser: false
  };
}

Answer 2

You need to make sure you have the correct context of the component when you use this :

componentWillMount() {
  var self = this;
  var requestEmail = this.props.email
  new Firebase(url)
    .orderByChild('email')
    .startAt(requestEmail)
    .endAt(requestEmail)
    .once('value', function (snap) {
      var foundUser = snap.val();
      if (foundUser) {
        self.setState({found: true})
      } else {
        self.setState({foundUser: false})
      } 
    });
},

Save a reference to this in a variable, or you could use .bind , or even better es6 arrow function, since it looks like you are using es6.

  new Firebase(url)
    .orderByChild('email')
    .startAt(requestEmail)
    .endAt(requestEmail)
    .once('value', (snap) => {
      var foundUser = snap.val();
      if (foundUser) {
        this.setState({found: true})
      } else {
        this.setState({foundUser: false})
      } 
    });