Description:

The description of String.prototype.matchAll says:

Each iteration result's value is an Array containing the results of the match, or null if the String did not match.

However, if the string did not match, the iterator returns undefined, not null, as far as you don't use an object with a custom @@matchAll property instead of the standard RegExp.

(Possibly this is because the old reivision of the proposal stated it returns null but the description remains unchanged?)

eshost Output:

$ eshost -s -x "const it = 'A'.matchAll(/B/g); const r = it.next(); print(r.done); print(r.value)"
#### JavaScriptCore, SpiderMonkey, V8, V8 --harmony, XS
true
undefined