The property 'navCtrl' cannot be read as it is undefined

I have been struggling for hours to find a solution to the issue in my project. Despite reading several other posts on the "Cannot read property ... of undefined" error, I am unable to resolve it.

Below is the relevant code snippet from my Ionic 2 / Apache Cordova project. The page mentioned is the Sign In Page for the app, which is not part of the core components but a regular page.

The problem lies in the inability of the NavController to be recognized within the onSignIn() method. Even though I have injected NavController in the constructor and followed all necessary procedures, the function fails every time without any clear reason.

import firebase from 'firebase';
import { Component } from '@angular/core';
import { NavController, NavParams } from 'ionic-angular';
import { HomePage } from './../home/home';

var provider = new firebase.auth.FacebookAuthProvider();

@Component({
  selector: 'page-signin',
  templateUrl: 'signin.html',
})

export class SignInPage {

  constructor(public navCtrl: NavController, public params: NavParams) {}

  onSignIn() {
    firebase.auth().signInWithPopup(provider).then(function(result) {
      console.log(result.credential.accessToken);
      console.log(result.user.displayName);
      this.navCtrl.setRoot(HomePage);
    }).catch(function(error) {
      console.log(error.message);
    });
  }
}

Answer №1

One effective approach to resolving this problem is by incorporating arrow functions:

An arrow function expression provides a more concise syntax compared to a traditional function expression and does not have its own bindings for this, arguments, super, or new.target.

To address your concern, simply modify the onSignIn method as follows:

  onSignIn() {
    firebase.auth().signInWithPopup(provider).then((result) => {
      console.log(result.credential.accessToken);
      console.log(result.user.displayName);
      this.navCtrl.setRoot(HomePage);
    }).catch((error) => {
      console.log(error.message);
    });
  }

Note the usage of (result) => {...} instead of function(result) {...}

Answer №2

Consider using the following code snippet:

let _currentInstance;

@Component({
  selector: 'page-signin',
  templateUrl: 'signin.html',
})

export class SignInPage {

  constructor(public navCtrl: NavController, public params: NavParams) {
    _currentInstance = this; //Place this line either here or in onSignIn method, depending on which one is called first
  }

  onSignIn() {
    _currentInstance = this; //Place this line either here or in the constructor, depending on which one is called first
    firebase.auth().signInWithPopup(provider).then(function(result) {
      console.log(result.credential.accessToken);
      console.log(result.user.displayName);
      _currentInstance.navCtrl.setRoot(HomePage);
    }).catch(function(error) {
      console.log(error.message);
    });
  }
}

The issue you are encountering may be due to the scope of the this keyword. In your scenario, the scope of this is likely within the function passed inside the then method.

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