Question

I know that there are better ways to check for an empty string—bear with me.

---

This is not checking for empty string in general. I’m actually doing something like:

var s = /*who knows?*/;
switch (s.charAt(0)) {
    // ...
}

and wanted to know if I can avoid having to do an extra if (!s).

Answer 1

Yes, as long as the value is a valid string. Section 15.5.4.4 of the language spec says

1. Let position be ToInteger(pos).
2. Let size be the number of characters in S.
3. If position < 0 or position ≥ size, return the empty String.

That was not changed since ES3. Interpreters are pretty good around string function compatibility.

That said, I recently implemented a peephole optimization for charAt to Closure compiler but I did not optimize out of bounds checks because the compiler tends not to optimize what are considered programmer errors.