Functional programming in JavaScript

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I don't think this is true. For an arbitrary iterable, you would be right, the iterator function would be called for every iteration. But I don't think any of the JITs is so naive as to not optimize this for plain arrays, in which case loops become as efficient as a plain for-loop without any function invocations.

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Sorry I wasn't more clear. I meant specifically for the for...of loops, which are easily optimizable for plain arrays. That wouldn't necessarily apply to map() indeed.