property/key orders are not guaranteed.
What You Should Know About ES6 Maps
Just Chris

Not by the spec, but I’m fairly certain Chrome had to modify their algorithm to provide a guarantee because every other browser vendor already did and Chrome was breaking the web by not conforming. So there maybe be no guarantee that it will remain in the given order for future browsers, but unless things have changed the current state is that every major browser actually returns them in FIFO order, except numeric keys which are returned first in numeric order. (and why would future browsers not jump on this convention anyway?) E.g.

for (var i in {b:6, a:7, c:4, 4:4, 1:1, 2:2, 3:3}) console.log(i)

Will print 1, 2, 3, 4, b, a, c in all major browsers.

Perhaps I’m missing something though.

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