我有一个this.state，我需要通过setState传入我的componentDidMount，如何在setState中使用bind（this）？

Question

I have that pass Bind in setState, how to do? enter image description here

TypeError: Cannot read property 'setState' of undefined

I am using the RealTime Firebase and ReactJs

constructor() {
    super()
    this.app = firebase.initializeApp(firebase_config);
    this.database = this.app.database().ref().child('users');
    this.state = {
        users: []
    }
}
componentDidMount() {
  this.database.once("value", function(snapshot) {
      snapshot.forEach(function(data) {
          this.setState({users: data.val()})
      })
  });
}

render() {
   return (
    <BrowserRouter>
       <div className="App">
          <Navbar/>
          <Switch>
            <Route exact="exact" path='/' component={CampaingListClients}/>
          </Switch>
          <p>{this.state.users}</p>
       </div>
    </BrowserRouter>);
    }
}

TypeError: Cannot read property 'setState' of undefined

Answer 1

You can resolve setState undefined issue in two ways

1. Change your functions to arrow functions

    componentDidMount() { this.database.once("value", snapshot => { snapshot.forEach(data => { this.setState({users: data.val()}) }) }); } 

2. Bind each function like below

     componentDidMount() { this.database.once("value", function(snapshot) { snapshot.forEach(function(data) { this.setState({users: data.val()}) }.bind(this)} }.bind(this)); }