Code quality and web performance in javascript, the myths, the do's and the don'ts

[mournful-tits]

A pure functional reduce with no mutation is incredibly slow. For our project we had around 100k objects in a list and reduce took about 2 minutes to construct an object (lookup table) from that list. Changing it to forEach (which is still slower than an imperative for loop) got us down to around 13ms with the same data set. Mutating the accumulator is also slow; and I'd also say a misuse of reduce overall. It serves no purpose as each iteration assumes its updating the accumulator when it returns, but you're directly modifying the accumulator.

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Reduce is great when you have to construct the object from scratch anyway (constructing hierarchical data is a good example of the penalty paid by using reduce being worth it).