Does ES6 Guarantee Ordered Object Property Enumeration?

Does ES6 Define an Enforced Order for Object Property Enumeration?

ES6 introduces well-defined property order for objects. However, it does not mandate a specific order for enumerating properties using operations like for-in, Object.keys, or JSON.stringify due to compatibility concerns.

ES2015-ES2019:

• for-in, Object.keys, JSON.stringify: No. These operations are not required to follow the object's property order.

• Other operations like Object.getOwnPropertyNames, Reflect.ownKeys: Yes (usually). These follow the defined property order:

  • Integer indices in ascending order
  • Other string keys in creation order
  • Symbol keys in creation order

ES2020 and Up:

As of ES2020, even legacy operations (for-in, Object.keys) are required to follow property order. However, relying on property order for program logic may still not be advisable as non-integer-index property order depends on creation time.

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