JavaScript - variables inside nested forEach loop through undefined

Question

I am trying to get the value of this.userId while inside the second for loop, it throughs undefined. Have a look at the code below:

 //the following variable gets undefined inside the second for loop
  this.userId = localStorage.getItem('userId');

  this.reviews.forEach(function(element1){

    element1.usersWhoLike.forEach(function(element2) {

   if(element2 == this.userId){
       //this throughs undefined !!
       console.log('UserID is : ',this.userId)
   }

  });

  });

`userId` is not the same as `this.userId`. – Raymond Chen Apr 23 '17 at 16:05 — Raymond Chen, Apr 23 '17 at 16:05

score 2 · Accepted Answer · answered Apr 23 '17 at 16:05

2

You missed scope of function inside forEach Let's try:

this.reviews.forEach(function(element1){

    element1.usersWhoLike.forEach(function(element2) {

   if(element2 == userId){
       //this throughs undefined !!
       console.log('UserID is : ',this.userId)
   }

  }, this);

  }, this);

answered Apr 23 '17 at 16:05

taile

2,738
17
29

It worked! But how ? – Hamza L. Apr 23 '17 at 16:10
1

@HamzaL. The 2nd argument given to `.forEach()` controls the value of `this` inside the iterator `function`s. Passing `this` as that argument ensures that all 3 functions (2 iterators and the function surrounding the snippet) use the same value. – Jonathan Lonowski Apr 23 '17 at 16:18
@JonathanLonowski Got it, thank you! – Hamza L. Apr 23 '17 at 16:24
For more detail. You can see the list parameters here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach?v=example#Parameters – taile Apr 23 '17 at 22:29

Noobit · Answer 2 · 2017-04-23T16:13:23.057

this.userId in the second one is referring to the function scope itself. Try adding a var self as a reference

    this.userId = localStorage.getItem('userId');

    var self = this; // Referencing
    this.reviews.forEach(function(element1){

        element1.usersWhoLike.forEach(function(element2) {

            if(element2 == userId){
                //this throughs undefined !!
                console.log('UserID is : ',self.userId)
            }

        });

    });

score 0 · Answer 3 · answered Apr 23 '17 at 16:12

0

this.userId = localStorage.getItem('userId');

Put this line inside first loop

answered Apr 23 '17 at 16:12

Osama

2,912
1
12
15

JavaScript - variables inside nested forEach loop through undefined

3 Answers3