Reflect.ownKeys has overly wide type definition

[jdalton]

Reflect.ownKeys has a return type definition of PropertyKey[] which is defined as
declare type PropertyKey = string | number | symbol; but the method of ownKeys only returns (string | symbol)[].

[fatcerberus]

To be fair, the type is not invalid, it’s just overly wide. The set of all strings is contained within the set of all strings and numbers, after all. Likewise if I have a function that always returns either “foo” or “bar” I can still claim it returns string despite not covering the entire set of possible strings.

Of course this is splitting hairs, but since you put “invalid” in the issue title I had to point it out 😉

A moderate part of the built-in types defines indexable types as string | number | symbol, when this doesn't exist in reality at all.

In actual runtime ECMAScript, numbers aren't properties of anything.

Examples:

JavaScript:
Arrays are often indexed by number, with the knowledge that the number is being implicitly cast to a string.

TypeScript:
Arrays must be indexed via number. This is great for safety and all; the chances that the string "fsfh654ewvbhy5" is a property of your array are less than that of the number 35 when cast to a string, but the properties aren't actually stored as numbers.

Note, this effects more than just Reflect.ownKeys. For example, all ProxyHandler functions that take an index value get string | number | symbol, when they should be string | symbol.

[ExE-Boss]

I’m fixing this in #41987.