I am creating this quick explanation of the differences between using .map() and .forEach(), because I originally had a difficult time understandin...
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Another thing I point out to devs new to javascript is the callback passed to
forEachis necessarily an impure function since it doesn't return anything.If a dev sees it in code they should assume it is mutating state.
Whereas, although
mapcan be abused to mutate state, since it always returns a new array, its design implies its callback is a pure function.mapandfilterare best treated as using pure functions.Because
forEachdoesn't work how the above two array methods work, I tend to avoid using it altogether and instead use the modernfor ... ofiteration approach when I need to mutate state.Thanks for writing this comparison. I've seen many devs get confused about this stuff 👍
Implies is a strong word... I'd say, "allows for". Per the ECMA standard, map() guarantees invocation order against the elements of the array, something that would be unnecessary to specify in a function that implies a pure function callback.
The standard itself explicitly says that while map doesn't mutate the object on which it is called, the callback function may mutate it (not just any old side effect here like console.log(), but a mutation of the object being called)... and there's an odd rule that if you delete an element from the array during the invocation of map(), but before map visits that element, that element will not be visited.
Sure, it's better style if it is pure, but the implication, if anything, is that it may be impure.
Those are good points.
In much of the code I see,
mapandfilterare used with pure functions.So, despite what the language allows, the use of them implies pure functions in my experience.
Following this pattern means you don't have to worry about the scenario you describe.
I wish that you have written this post just a week earlier. My life would be so much easier. Amazing post!
I'm sorry I wasn't there in time to save you! haha.
thank you very much, Victoria, for your extremely clear explanation. I would say that a functional programming style is ever a good choice, so I vote "map" for President! Have a good week end :)
You're welcome and thank you for commenting and adding some lovely humor. haha! I hope you also have a great weekend.
Thank you so much for explaining it this way. I've sort of always understood the difference but now it makes a lot more sense.
Hi Victoria, thank you so much for this, it definitely helped me understand both methods a lot better. One thing i'm having difficulty understanding is how forEach is able to push the result into the result array, if it returns undefined?
If I was a JS newbie I would have needed this
Oh wow thank you! I've always completely ignored learning map in any language because I had foreach but this is a great straight forward explanation
Time to look over my old code 😁
Also in java forEach is a terminal operation that closes the stream when it finishes. In the other hand, the map as in js it returns a new stream with the result of the mapping.
Really clearly explained, well done!
Thank you!!
I think I will tweet this post a few more times.