Most projects do just use straight JavaScript which is a big problem. JavaScript does all the wrong things by default. "for in" loops and "==" comparisons are a

Most projects do just use straight JavaScript which is a big problem. JavaScript does all the wrong things by default. "for in" loops and "==" comparisons are a couple big offenders, that often appear to do the right thing (but fail later in unexpected ways). Any of these compilers worth their salt will limit their output to properly formed loops and comparisons and otherwise cross-browser compatible code, etc, saving you from a great deal of pain.