Undefined is not a function this.setState

问题

I created a simple login page, which, once the button is pressed, executes this function :

login = (email, password, navigate) => {
  this.setState({ loginButtonPressed: true });
  firebase
    .auth()
    .signInWithEmailAndPassword(email, password)
    .then(function(user) {
      navigate('Profile');
    })
    .catch(function(error) {
      this.setState({ loginButtonPressed: false });
      Alert.alert(error.toString());
    });
};

Once called, it executes "this.setState({ loginButtonPressed: true })" since the button changes shape (it is marked as pressed). But I get the following error: Undefined is not a function this.setState.

How can this be fixed? Thanks for your help.

回答1:

The problem is with:

.catch(function(error) {
  this.setState({ loginButtonPressed: false });
  Alert.alert(error.toString());
});

you create an anonymous function where this will reference this anonymous function (Object), not a React Object.

I see you can use arrow functions, so fix it like:

.catch((error) => {
  this.setState({ loginButtonPressed: false });
  Alert.alert(error.toString());
});

With arrow function this will reference context in which it is used.

回答2:

Or you can keep a copy of the reference to this, like this:

const self = this; (first line in your function);

and use "self" instead of "this" anywhere in the function.

You can use this in places where you might not be able to use arrow functions.

回答3:

You need to bind to this in order to have access inside the callback. Try it with the arrow functions.

回答4:

Are you extending react component?

If you are not you will need to do that to utilize setState, if you are you will need to be sure to create the state in a constructor before you update it with setState elsewhere.

Something along the lines of:

class Login extends React.Component {
  constructor(props) {
    super(props);
    this.state = { loginButtonPressed: false };
    this.login = this.login.bind(this);
  }

  login(email, password, navigate) {
    this.setState({ loginButtonPressed: true });
    firebase
      .auth()
      .signInWithEmailAndPassword(email, password)
      .then(function(user) {
        navigate('Profile');
      })
      .catch(function(error) {
        this.setState({ loginButtonPressed: false });
        Alert.alert(error.toString());
      });
  };

  render() {
    return (
      <Button onClick={this.logIn}>Login</Button>
    );
  }
}

回答5:

An arrow function remembers it lexical scope. Refer MDN documentation -- "An arrow function does not newly define its own 'this' when it's being executed in the global context; instead, the 'this' value of the enclosing lexical context is used, equivalent to treating this as closure value. Thus, in the following code, the 'this' within the function that is passed to setInterval has the same value as 'this' in the enclosing function:" function Person() { this.age = 0;

  setInterval(() => {
    this.age++; // |this| properly refers to the person object
  }, 1000);
}

var p = new Person();

来源：https://stackoverflow.com/questions/49821672/undefined-is-not-a-function-this-setstate