You can modify quite a few objects in JavaScript, but you can’t modify JavaScript using eval. There’s no way to add an operator to it, for example (AFAIK. I’m not following its rapid evolution closely)

If you want that, use Smalltalk, a Lisp or a Forth.

Also, the brain does modify itself at least as much as you can modify JavaScript. For example, my brain nowadays accepts that 0.999999… is equal to 1, that there are equally many primes as natural numbers, etc.