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Comment by jongjong | Hacker News Reader

I don't buy these arguments about 'This cheap computation is 6x faster than this other alternative cheap computation'.

For example, in JavaScript, using standard 'for loops' with index numbers is a LOT faster (over 16 times faster for basic use cases) than using Array.forEach(), yet all the linters and consultants recommend using Array.forEach() instead of the standard for loop... What about latency??? Suddenly nobody cares about latency nor performance.

The reason is that these operations which use marginal amounts of resources are pointless to refactor. If a function call which uses 1% of CPU time* (to service a standard request) has its performance improved by 'a whooping' 50%, then the program as a whole will use only 0.5% less CPU time than it did before.

* (where all the other operations use the remaining 99%)

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> For example, in JavaScript, using standard 'for loops' with index numbers is a LOT faster (over 16 times faster for basic use cases) than using Array.forEach(), yet all the linters and consultants recommend using Array.forEach() instead of the standard for loop... What about latency???

Do you have proof of this claim? It smells like bs.

Not the person you are asking but, 16 times seem like a lot, however it's going to heavily depends on how good the VM or JIT is at inlining.

Because `Array.forEach` is one method call for each iteration, when `for loops` stay on the same frame. If the compiler can't inline that, it's a "major" overhead compared to simply jumping back at the top of the loop.

Googling for it I find some benchmark showing a 3x performance: https://leanylabs.com/blog/js-forEach-map-reduce-vs-for-for_..., but they call `Array.push` in the loop, so it's possible the difference is even bigger in practice.

forEach() incurs a function-call overhead unless you optimize it away in a compiler. So the idea a loop would be faster by some multiplier is quite plausible.

To get an order of magnitude in difference, you'd have to construct a special case. I've seen a multiplier of about 3, but not 16.

As an aside: you use forEach() over for loops because most array processing operates on small arrays, where the marginal improvement of using a loop is limited. If you have a large array, you will eventually switch to a for loop if the loop body is relatively small. Likewise, when the request size is small, JSON works fine. But when your requests grows in size, the JSON overhead will eventually become a problem which needs a solution.

The underlying consideration is Amdahl's law.

If you have Node.js installed, you can run it yourself. I did run the experiment, that's how I came up with the 16x number. I just had the two kinds of loops iterating about 10 million times and just assigned a variable inside them (in the same way). I recorded the start and end times before and after the 10 million iterations (in each case) and found that Array.forEach took 16x longer to execute. The difference could be even higher if the operation inside the loop was cheaper since much of the cost of the loop depends on what logic is executed inside it. I kept it light (variable assignment is a cheap operation) but it's still an overly generous comparison. The real difference in performance of just the two kinds of loops are almost certainly greater than 16x.

Ideally you should compare running the loops without any logic inside them but I was worried that optimizations in the JavaScript engine would cause it to just skip over the loops if they performed no computations and without any memory side effects. Anyway this was beside the point I was trying to make. My point is already proven; it makes no sense to optimize cheap operations.

forEach is faster than indexing for me on Firefox Android. Tried it with this: https://www.measurethat.net/Benchmarks/Show/13207/0/performa...

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