> The most important thing is that there is some defined order. This makes it easier to debug, and reproducibility is just a very nice property. See https://en.

> The most important thing is that there is some defined order. This makes it easier to debug, and reproducibility is just a very nice property. See https://en.wikipedia.org/wiki/Reproducible_builds https://en.wikipedia.org/wiki/Reproducible_builds Simple stuff like making golden output files for your tests becomes easier. Relying on this type of behavior when it isn't required is somewhat of an anti-pattern. And can often lead to mindlessly copying the results of an implementation in tests. A quick aside: any sort of generic ordered container that supports O(n) iteration must necessarily require O(n log n) work to construct in the average case. So the asymptotic complexity is necessarily the same as sorting the keys of a hash map with undefined iteration order to artificially create a defined order. It isn't too hard to simulate the behavior of insertion order iteration by maintaining an array alongside the hash map. And surprisingly enough, space and runtime complexity is practically identical to containers that implement insertion order iteration ;-)