I wrote a javascript compiler. At the time (2000), it was twice as fast as Microsoft's and 20 times faster than Netscape's. I was correct in anticipating that js speed would become very important, but my implementation was ahead of its time and didn't get any traction.
It has since been rewritten in D and is Open Source:
License it to one of the browser vendors. I did do a few licenses, but the trouble was I knew how to reach a mass market, but the market for js engines was small and only large companies. I struggled with that.
It's still quite a good scripting engine, if you want to, for example, have scripting in your IDE so users can extend it. JS is a good language for that.
It has since been rewritten in D and is Open Source:
https://github.com/DigitalMars/DMDScript