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.

Answer 1

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.