Rather, there is the ‘specification’ of the language (the ECMAScript standard), and then there are many ‘implementations’ of that standard. These are generally quite separate projects made by separate people. Many popular programming languages are developed in a similar way, having a variety of implementations of a single standard.
It’s possible that a language may have an open source implementation, but the standard may not be “open” — it’s changes may be controlled by a single corporate entity for its profit. Or, the standard may be “open” and developed collaboratively, but there may be only one implementation which is not open source, or, the only “good” implementations may be proprietary.
Because of its open standard, ECMAScript is open but not open source. (A language standard can not be open-source — it is not a program, it is a document describing the expected behavior of a program — but an implementation of it can be.)