Why coerce the separator to a string?

[linusg]

The proposal does not predate the guidance at https://github.com/tc39/how-we-work/blob/main/normative-conventions.md#avoid-coercing-arguments-to-types-other-than-boolean and therefore should follow it.

[ljharb]

None of these rules are inviolable, but you should have a good reason for any particular feature to deviate (for example, because the feature is a close cousin of an existing feature, like findLastIndex vs findIndex).

I don't have a strong preference for coercing or not, here, but the existing uses of join that do coerce would be sufficient justification for coercing here, I think.

[linusg]

Iterator.prototype methods already diverge from their Array and TypedArray counterparts by not supporting a thisArg and passing a different number of arguments to callbacks.