Is it acceptable style for Node.js libraries to rely on object key order?

No. Rely on the ECMAScript standard, or you'll have to argue with the developers about whether a "de facto standard" exists like the people on that bug.

Definitely do not rely on the order of the keys. If the standard doesn't specify an order, then implementations are free to do as they please. Hash tables often underlie objects like these, and you have no way of knowing when one might be used. Javascript has many implementations, and they are all competing to be the fastest. Key order will vary between implementations, if not now, then in the future.

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.