I tried this in your online demo with constant evaluation enabled:

console.log(2 + 2 * 2);
console.log('abcdefgh'[4]);
console.log({a:'b'}['a']);

The result is:

console.log(6),console.log("abcdefgh"[4]),console.log({a:"b"}.a)

Only the first (numeric) expression was optimized. I would expect all of them to be evaluated.

This should be a safe transformation if only someone didn't override Array or Object constructor. (Note that accessing non-plain object's properties is not safe as they might have a getter.)