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Comment by n8agrin | Hacker News Reader

In the context of a large client application, I often advise engineers that if we're optimizing things like the types of `for` loops we use, we've won the performance lottery. That is, I've never found a critical performance issue that is the result of using `forEach` instead of `for`.

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Agreed. I know that in some small use cases these differences are crucial, but in 95% of situations arguing these differences just feels like a waste of time.

I had the opposite experience. We had a large client application that was too slow, with no obvious bottleneck on the flame graph. I replaced all the functional iterators by for loops among other similar optimisations, and improved the performance by a factor of 50. If you use programming constructs that are 50 times slower on average, your program will be 50 times slower on average.

> If you use programming constructs that are 50 times slower on average, your program will be 50 times slower on average

Well, yes, but the relative difference between a for and foreach loop is miniscule - not 50x difference. In absolute terms, the overhead of both is barely measurable, and is extremely unlikely to be a bottleneck in any client-side application.

I've had it happen once, sort of, on a relatively small collection. Several seconds' delay due to the use of foreach, that were actually annoying production users.

The issue was that the collection being iterated over was a non-generic .NET 1.0 DataTable. Using a foreach loop would implicitly box and then re-cast each object, while the for loop directly accessed the correctly typed .Item() and did not need to do that.

Ironically, the body of the loop was a fairly tricky logistics algorithm I had just written, so I had every reason to assume the problem was on the inside. Imagine my surprise when I changed it to a for loop - strictly to access the index and print out some timings - and watched the procedure suddenly become instant...

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