Javascript is too dynamic to have a safe subset. For example, using only the six characters ()+ []! you can write arbitrary code. The main culprits are the weak typing, permissive attribute access, and large runtime environment with lots of surface area. This is unlikely to be fixable by changing the language grammar.

See http://www.jsfuck.com/

    JSFuck is an esoteric and educational programming style
    based on the atomic parts of JavaScript. It uses only six
    different characters to write and execute code.
    It does not depend on a browser, so you can even run it on
    Node.js.