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

Josh Ellis on October 09, 2020

I've been doing a lot of algorithm practice lately, and I just came across a quirk of how for...in works. I love using JavaScript's for...in and f...

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lionel-rowe • Oct 9 '20 • Edited

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.

Josh Ellis • Oct 9 '20 • Edited

An alternative that I didn't want to get into is using the index param from arr.forEach, but it is an option:

arr = [1, 2, 3]
arr.forEach((el, idx) => {
    n = idx + 5
    console.log(n) // 5, 6, 7
})

Srikanth • Oct 9 '20

It's informative and straight to point..
Also if possible can you change examples for "What is for...in?" as

array = [4, "some text", true];

for (let i in array) {  //  in for..in 'i' returns index
  console.log(i); // 0, 1, 2
  console.log(typeof i); // string, string, string
  console.log(array[i]); // 4, some text, true
}

for (const el of array) {   //  in for..of 'el' returns element
  console.log(el);  // 4, some text, true
  console.log(typeof el);   // number, string, boolean
}

Anyways..thanks for the information

Josh Ellis • Oct 9 '20

Thanks for the extra info!

Vamp • Oct 9 '20 • Edited

the reason for this is that “for...in” iterates over all the enumerable (own, not proto) properties of the object. this means that the array object (subject) will have its properties (index values) iterated over.

the reason they are a string is because all object properties are stored as strings. the properties of an array are indices which are presented as strings just as any other object properties.

you can use Object(arr).entries as the iterator which will give you [index, element] pairs in “for...of”. because “for...of” iterates over the elements themselves which will be those entry pairs.

Josh Ellis • Oct 9 '20

Thanks for the extra details! Makes sense that is just like object properties note that you mention it

leob • Oct 10 '20 • Edited

Good read ... who knows a mnemonic or aid to help remembering the difference between "for in" and "for of"? because I always forget which is which ...

Ah got it already - I think this would work:

for "I"n yields "I"ndexes
for "O"f yields "O"bjects

So "for In" yields the "I"ndexes (keys) of the elements, "for Of" yields the "O"bjects (the elements themselves) ... playing it a bit fast and loose, but you get the idea ;-)