[pluma]

The reason ES4 was killed was that there was no consensus between the stakeholders. It would have been a breaking change, making conforming engines incompatible with all existing JavaScript code.

How well major breaking changes like that work out on the web can be seen by the massive success (cough) of XHTML -- I dare you find me one notable site correctly using XHTML with the appropriate MIME type (rather than just sending XHTML as HTML tagsoup using `text/html`).

Web Assembly works because it is not a breaking change to JavaScript. It's actually defined in a way that is mostly orthogonal to JavaScript -- it shares some of the APIs but most likely there will be (low-level) APIs only Web Assembly code can use and some (high-level) APIs that only exist for JS.

[pier25]

I'm not advocating for breaking changes, in fact static type checking in ES4 was optional, the language remained just as dynamic if you wanted to use it that way.