With a compilation step, you can have error messages that developers see, but users don't. (Hopefully, these errors enable the developers to reduce the errors that users see, and just generally improve the UX, but that's NOT guaranteed.)

Without a compilation step, you have to assign some semantics to whatever random source string your interpreter gets. And, while you can certainly make that an error, that would rarely be helpful for the user. JS instead made the choice to, as much as possible, avoid error semantics in favor of silent coercions, conversions, and conflations in order to make every attempt to not "error-out" on the user.

It would be a very painful decade indeed to now change the semantics for some JS source text.

Purescript is a great option. Typescript is okay. You could also introduce a JS-to-JS "compilation" step that DID reject (or at least warn the developer) for source text that "should" be given an error semantic, but I don't know an "off-the-shelf" approach for that -- other than JSLint.