Discussion on: Problems with Using for...in on Arrays in JavaScript

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You can also use Array#keys to get an iterator with the array indexes as numbers:

const arr = [1, 2, 3]

for (const i of arr.keys()) {
    console.log(typeof i, i) // number 0, number 1, number 2
}

Similarly, Array#values gives the elements of the array, and Array#entries gives tuples of [index, element]:

for (const [idx, el] of arr.entries()) {
    console.log({ idx, el }) // {idx: 0, el: 1}, {idx: 1, el: 2}, {idx: 2, el: 3}
}

Josh Ellis • Oct 9 '20 • Edited

Awesome! I didn't put 2 + 2 that .keys() and .entries() would work on arrays (I use them all the time for objects), that's great.

lionel-rowe • Oct 9 '20

It's a little different from Object.keys and so on. You can do Object.keys(arr) too, but then you'll get an array of string keys, rather than an iterator of number keys. Interestingly, there's no such thing as Object#keys — I guess it doesn't make sense to return an iterator of keys for something that itself isn't iterable.

Josh Ellis • Oct 9 '20

Right, because they're class methods on Object that take a param, that's probably why I didn't think to look into how it could work with arrays. Very interesting.