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Discussion on: Stop mutating in map, reduce and forEach

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Shon Vella

You first example feeling about map being used where forEach should have been is correct, but you seem to not understand why other than it seems wrong. The reason map is inferior in this example is that it will create a new array and fill it with the return value of each call to checkID() (probably undefined) and then immediately throw it away. Memory allocation (and the subsequent associated garbage collection) isn't free and in fact probably more expensive by itself than what checkID() is doing.

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Stephan Meijer Author • Edited on

I'm well aware of the fact that a map creates an array, and that the return value is not used. That's just not the problem I wanted to focus on in this article. I could have just as well made the contrived examples a bit more complex, by creating an example that does use the return value from that map function, while also executing a side effect. But that would have made the examples harder to grasp, while it doesn't contribute to the message I was trying to send.

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Shon Vella

Sorry, didn't mean to offend. You did specifically say "It was just that it doesn't match with my expectations", which lead me to believe you were going off of a gut feeling rather than a concrete pragmatic reason for raising it as an issue, which in turn lead me to lose confidence in what you had to say and I stopped reading.

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Stephan Meijer Author • Edited on

No. It's not just gut feeling. Or well, maybe the real request for change was. Because I do understand what you're saying, but the performance impact there is so minimal, that I would not use that as argument to request a change.

I find the readability issue (unexpected side effects) a way bigger problem.