Does ES6 Guarantee Consistent Property Enumeration Order Across All Operations?

Deterministic Property Enumeration in ES6

Question:

Despite introducing property order, does ES6 guarantee a well-defined order for enumerating object properties in all operations?

Answer:

ES2015-ES2019:

No, for certain operations such as for-in, Object.keys, and JSON.stringify, ES6 does not enforce a specific order of enumeration. This is due to backward compatibility concerns.

for-in loops, which utilize the [[Enumerate]] method, have undefined enumeration order as per the ECMAScript specification.

Object.keys also allows implementations to define custom enumeration orders for for-in statements.

ES2020 and Later:

Starting with ES2020, property order must be respected even for legacy operations like for-in and Object.keys.

Additional Operations with Defined Order:

Some other operations in ES6, such as Object.getOwnPropertyNames, Object.getOwnPropertySymbols, and Object.defineProperties, follow property creation order for ordinary objects. This order is:

1. Integer indices (numerically ascending)
2. String keys (property creation order)
3. Symbol keys (property creation order)

Exceptions:

However, certain exotic objects, such as Proxies, may define their own [[OwnPropertyKeys]] method to modify enumeration order.

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