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PHP has had this undocumented feature since forever and so did all mainstream Javascript engines.

Big whoop

> Javascript

Javascript doesn't specify performance characteristics of objects and arrays, so even with implementations, one object could be a hashtable, another a balanced tree.

The implementation doesn't matter to EGreg's point, which is that ES standardized that iterating over object keys are generally required to return keys in insertion order.

In the past (from before this standardization) Chrome had in fact changed the object iteration order due to an optimization, and had to revert it after lots of complaining on their bug tracker.

(To be precise, the spec still requires array index properties to be returned ahead of other properties regardless of insertion order. The behavior that Chrome "reverted" to is this new one, so not exactly the same as the original behavior.)

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