Why must ProxyHandler’s `ownKeys(…)` method’s return value contain only strings or symbols?

[ExE-Boss]

A lot of JavaScript user code assumes that you can pass arbitrary values where JavaScript expects property keys.

Examples:

• Object.defineProperty(…)
• Object.getOwnPropertyDescriptor(…)
• Reflect.defineProperty(…)
• Reflect.deleteProperty(…)
• Reflect.get(…)
• Reflect.getOwnPropertyDescriptor(…)
• Reflect.has(…)
• Reflect.set(…)

This makes [[ProxyHandler]].ownKeys(…) into a footgun, as user code might assume that the contents of the returned array‑like will be processed by ToPropertyKey(…), instead of throwing when it encounters a value that is neither a string, nor a symbol, like it does currently.

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See also: microsoft/TypeScript#35594

[TimothyGu]

Without historical context on how the decision was made, it could be seen as an instance of robustness principle (Postel's law): the functions that you listed (consumers of property keys) are liberal in what they accept, but we require the proxy hooks to be conservative in what they produce.

[ljharb]

cc @erights @tvcutsem