如何从另一个函数内部的 firebase 函数内部访问一个函数

Question

I have a piece of code where there are three functions. One function is go(), another one is checkIfUserExists() and another one is userExistsCallback(). The go function calls the checkIfUserExists function. Insider checkIfUserExists function I call a firebase function which then needs to call userExistsCallback(). But I am not being able to access userExistsCallback from inside that firebase function.

async go() {

  var userId = 'ada';
  this.checkIfUserExists(userId); // this is working. It perfectly calls the function
  console.log('the go function');
}


async userExistsCallback(userId, exists) {
  if (exists) {
    console.log(' exists!');
 } else {
    console.log(' does not exist!');
  }
  console.log('function userExistsCallback ends');
}


async checkIfUserExists(userId) {

  var usersRef = firebase.database().ref("news/");
  usersRef.child(userId).once('value', function(snapshot) {
    var exists = (snapshot.val() !== null);
    this.userExistsCallback(userId, exists); // this is not working. 
    console.log('function checkIfUserExists');
  });

}

Answer 1

this is not working because it refers to the enclosing function , in this case your once callback.

Change your once callback to an arrow function which doesn't bind this and you're good to go:

async checkIfUserExists(userId) {

  var usersRef = firebase.database().ref("news/");
  usersRef.child(userId).once('value', (snapshot) => {
    // `this` now refers to `window` where your global functions
    // are attached.
    var exists = (snapshot.val() !== null);
    this.userExistsCallback(userId, exists); // this is not working. 
    console.log('function checkIfUserExists');
  });
}