Object prototype can't access properties with `this`

Question

So, I saw several examples that this should work. But obviously, I'm missing something, because it doesn't. :/

Could somebody please explain to me what I'm doing wrong here? :)

function Code (b) {
  this.b = b
  this.arr = []
}

Code.prototype.add = (v) => {
  console.log(this.b)
  this.arr.forEach(element => {
    console.log(element)
  });
  this.arr.push(v)
}

var c = new Code('bla')
console.log(c.add('asdf'))

So this throws an error with:

this.arr.forEach(element => {
  ^

TypeError: Cannot read property 'forEach' of undefined

Obviously, I'm doing something wrong here. But I don't know what.

Thanks! Gergely.

arrow (or rocket) functions `this` is not the same as regular functions `this` - in general, arrow functions are not the right pattern for an objects prototype functions — Jaromanda X, Dec 08 '17 at 05:31
I agree with @JaromandaX. Change your code from this: `Code.prototype.add = (v) => {` to this `Code.prototype.add = function(v) {` That will correct the `this` pointer. Using an arrow function *forces* the `this` pointer to be what it was at the time the function was declared. — Intervalia, Dec 08 '17 at 05:33

score 2 · Accepted Answer · answered Dec 08 '17 at 05:34

2

function Code (b) {
  this.b = b
  this.arr = []
}
Code.prototype.add =function(v){
  console.log(this.b)
  this.arr.forEach(function(element){
    console.log(element)
  });
  this.arr.push(v)
  console.log(this.arr)
}

var c = new Code('bla')
console.log(c.add('asdf'))

You should use function(), instaed of () => arrow fucntion, since arrow function's this works differently.
this of an arrow function only refers to this that exists in the outer scope.

answered Dec 08 '17 at 05:34

moon

640
6
14

2

Just a pointer, it is a bad practice to answer a duplicate. If you know the issue, you are more likely to form better search query and find a match. You should look for such posts and mark post as duplicate. I also see, you do not have enough rep for it, so you should add the link in comment and someone will vote. Lets try to keep the portal clean. Have a good day :-) – Rajesh Dec 08 '17 at 05:37
@Rajesh Thank you for your kind explanation on how to use this site well! – moon Dec 08 '17 at 05:46
DAMN IT! I KNOW I tried this. :( That's why I didn't see for the duplicate. And nooow of course it works. Like... Wth man. :( Anyways. Thank you! :) – Hannibal Dec 08 '17 at 06:39

score -1 · Answer 2 · answered Dec 08 '17 at 05:34

-1

function Code (b) {
  this.b = b
  this.arr = []
}

Code.prototype.add = function(v) {
  console.log(this.b)
  this.arr.forEach(element => {
    console.log(element)
  });
  this.arr.push(v)
}

var c = new Code('bla')
console.log(c.add('asdf'))

answered Dec 08 '17 at 05:34

Madhavan.V

825
1
6
14

2

Just a pointer, it is a bad practice to answer a duplicate. If you know the issue, you are more likely to form better search query and find a match. You should look for such posts and mark post as duplicate. I also see, you do not have enough rep for it, so you should add the link in comment and someone will vote. Lets try to keep the portal clean. Have a good day :-) Also, an answer without explanation is incomplete. – Rajesh Dec 08 '17 at 05:37
Okay I understood. I will follow that next time. Thank you and wish you the same. :) – Madhavan.V Dec 08 '17 at 05:41

Object prototype can't access properties with `this`

2 Answers2