Discussion on: JS Array Manipulation Quirks

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forEach doesn't mutate an array. Mutation happens only if you explicitly mutate array items in iteration function.

I guess if you're using forEach like map you could say it mutates

arr.forEach(a => a = {id: a})

// VS

const newArr = arr.map(a => {id: a})

// VS

const newArr = [];
arr.forEach(a => newArr.push({id: a}))

Eugene Karataev • Dec 8 '19

Well, actually if we execute your examples, we can notice that forEach doesn't mutate the original array.

Jen Chan • Dec 8 '19 • Edited

Thanks for pointing that out. This seems like a gray area. I don't know how I got under the impression that forEach mutates the original array items, maybe it was this post I looked up as I was writing the post. It's not entirely clear to me what forEach actually returns if I ran it on an array its own.

I have around the web read general opinions that forEach iterates straight through arrays without breaking, and skips empty values, so that if one wanted to include a condition it would be easier to go with a for-loop. I don't have enough experience with forEach to tell, tbh

Francesco Belvedere • Dec 8 '19

To address your point "what forEach actually returns", the answer is "nothing". forEach does not return a value, it is a void function.

Jen Chan • Dec 9 '19

Thank you! That makes a lot of sense.