I have one this.state and i need pass in my componentDidMount with setState, how use bind(this) in setState?

问题

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

回答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));
      }
   

来源：https://stackoverflow.com/questions/54615829/i-have-one-this-state-and-i-need-pass-in-my-componentdidmount-with-setstate-how