It is possible to assign array 'value key' to a value?

Question

I have an array like this

var list =['lemon', 'apple'];
list['yuck'] = 'durian';

list.forEach(function(i){
    console.log(i);
});

The out put is

lemon
apple

So where is durian?

This question is not about how use array properly, I just find it odd that we able to do that, what's really going on here?

Your adding a property `yuck` to the array. Not adding a new index. `forEach` only iterates the arrays indexes. Index's can only be integers greater than zero. There are a few other rules too like max size and the relationship between them and the `length` property. — ste2425, Apr 08 '16 at 11:08
Too bad the good answer in the linked QA isn't accepted. There's still no point in not closing this one. — Denys Séguret, Apr 08 '16 at 11:10
this is a perfectly valid question and doesnt need the down votes — Oliver Watkins, Apr 08 '16 at 11:10
@OliverWatkins I agree downvotes aren't necessary. It needs a close vote though as it's a duplicate. — Denys Séguret, Apr 08 '16 at 11:11
i would say they are similar but not the same. This question is more succinct and should replace the older question — Oliver Watkins, Apr 08 '16 at 11:12

Denys Séguret · Accepted Answer · 2016-04-08T11:14:22.533

2

forEach iterates on an array's elements, that is the properties identified by a positive integer. 'yuck' is a string which can't be converted to an integer.

If you really want to iterate on all values, you can do this:

for (var key in list) {
  console.log(list[key]);
}

But it breaks all the optimizations of arrays. You should probably use a Set or an object instead.

edited Apr 08 '16 at 11:14

answered Apr 08 '16 at 11:08

Denys Séguret

I checked out set, it's not full supported yet at this moment. – angry kiwi Apr 08 '16 at 11:26
If the browser you're targeting don't support it, then use a simple object instead of an array. Or use push to add elements. – Denys Séguret Apr 08 '16 at 11:27
can you please elaborate why it breaks optimization? – angry kiwi Apr 08 '16 at 11:28
Array manipulation are very fast in moderne JS engines when the array is used in an expected way. You can see an introduction to this problem here: https://github.com/petkaantonov/bluebird/wiki/Optimization-killers (with exemples less basic than just making an array not an array) – Denys Séguret Apr 08 '16 at 11:32

score 2 · Answer 2 · answered Apr 08 '16 at 11:08

2

Because that's not how you add a value to a javascript array. You have to use the method Array.prototype.push()

var list = ['lemon', 'apple'];
list.push('durian');

list.forEach(function(i){
    console.log(i);
});

answered Apr 08 '16 at 11:08

Vitor Rigoni

score 1 · Answer 3 · answered Apr 08 '16 at 11:11

list['yuck'] = 'durian';

Here the syntax says add the text durain to a property yuck.

Here you are adding key values. This is how you assign values to a object.

The above line will throw an exception as you cannot add key values to array. So your array is never modified.

And this one

list.forEach(function(i){ console.log(i); });

You are looping the initial array. Hence same values are printed.

On other hand you can use. list.push('durian')

Or

list[2] = 'durian'

3 Answers3