Question

Enumerating the keys of javascript objects replays the keys in the order of insertion:

> for (key in {'z':1,'a':1,'b'}) { console.log(key); }
z
a
b

This is not part of the standard, but is widely implemented (as discussed here):

ECMA-262 does not specify enumeration order. The de facto standard is to match
insertion order, which V8 also does, but with one exception:

V8 gives no guarantees on the enumeration order for array indices (i.e., a property
name that can be parsed as a 32-bit unsigned integer).

Is it acceptable practice to rely on this behavior when constructing Node.js libraries?

Answer 1

Absolutely not! It’s not a matter of style so much as a matter of correctness.

If you depend on this “de facto” standard your code might fail on an ECMA-262 5th Ed. compliant interpreter because that spec does not specify the enumeration order. Moreover, the V8 engine might change its behavior in the future, say in the interest of performance, e.g.