In theory it would be possible to write a specification for a pure subset of JavaScript. But it's very important to acknowledge that this pure subset doesn't exist, and won't exist unless and until such a specification is written, to the point that it's possible to write an automated tool that can verify whether a given piece of code conforms to this specification. And if such a specification did exist, we would likely call it a "language" in its own right.

Once you write that language and confirm that your library conforms to it you can say your library is pure. Saying your library is pure before you've actually formally checked it is like saying your library doesn't have any bugs in before you've made any attempt to look for them - i.e. almost certainly a false, and so irresponsible to claim that it qualifies as a lie in my book.