In a similar vein to "use strict" we could create a new directive that disallows JS features like var, prototype, == (in favor of ===). Modules with this new directive at the top of the file would be executed under these new rules. You'd be able to create new (maybe lighter weight) JS interpreters that only target this subset.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...