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

ES6 Property Order: Is It Guaranteed for All Operations?

Problem:

ES6 introduces property order for objects. But does this new feature guarantee a well-defined order for all object property enumeration operations, such as for-in loops and Object.keys?

Answer for ES2015-ES2019:

No, not for all operations.

For for-in loops, Object.keys, and JSON.stringify, ES6 does not require a specific order of enumeration. Legacy compatibility concerns have influenced this decision.

For [[Enumerate]], which is used by for-in loops, the order of enumeration is unspecified. Object.keys also inherits this behavior.

Note: ES2020 subsequently changed this behavior, requiring even legacy operations to follow property order.

Other Operations and Property Order:

While the aforementioned operations lack a defined enumeration order, other operations do adhere to property order. These include:

• Object.getOwnPropertyNames
• Object.getOwnPropertySymbols
• Object.defineProperties
• Reflect.ownKeys

For these operations, the order is as follows:

1. Integer indices in ascending order
2. Other string keys in property creation order
3. Symbol keys in property creation order

Exception: Exotic objects, such as Proxies, can override the [[OwnPropertyKeys]] internal method and return keys in a different order.

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