Does ES6 Guarantee Consistent Object Property Enumeration Order Across All Operations?

Does ES6 Establish a Unifying Object Property Enumeration Order?

While ES6 introduces the concept of object property order, the question of whether it imposes a well-defined order for property enumeration remains. This article explores the nuances of this issue and examines how different operations behave in this context.

ES2015-ES2019: Limited Order Guarantees

For operations like for-in, Object.keys, and JSON.stringify, ES6 does not mandate a specific property order. Legacy compatibility concerns dictate this decision. For-in loops iterate based on the [[Enumerate]] internal method, which allows for unspecified order.

Specific Order Operations

Certain operations, including Object.getOwnPropertyNames, Object.defineProperties, and Reflect.ownKeys, do follow a predefined order for ordinary objects. This order proceeds as follows:

1. Integer indices in ascending order (if applicable).
2. Other string keys in property creation order (if applicable).
3. Symbol keys in property creation order (if applicable).

Exceptions for Exotic Objects

It's important to note that exotic objects, such as Proxies, can define their own [[OwnPropertyKeys]] method and override the default order. This behavior underscores the fact that the predefined order is not universally guaranteed.

Conclusion

ES6 introduces property order but grants limited guarantees for certain operations due to legacy compatibility. While some operations follow a predefined order, others continue to exhibit unspecified enumeration order, leaving developers with the responsibility to handle property ordering explicitly when necessary.

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