Stage 4 JS proposal: String.prototype.matchAll

[bakkot]

This proposal got stage 4 at the March 2019 meeting, and is not yet implemented in js-sys.

See spec text and MDN docs.

Note that this will also require adding Symbol.matchAll (spec text, MDN docs). In theory it might also require adding RegExp.prototype[@@matchAll], but I see the other RegExp.prototype[symbol] methods are also missing (@@match, @@replace, @@search, @@split), so perhaps that could be addressed separately.