JS is an accident of history, and won because of market quirks rather than technical merits. JS was created because Netscape thought it would be cool to have webpages that were somewhat interactive. They were a relatively small and chaotic startup throwing shit at the wall; MS did what MS does, which was to embrace / extend / extinguish, and by doing so they established JS as a must-have feature for all browsers. Before GMail and GMaps, JS was not particularly successful - it was largely just a novelty. Seriously-interactive stuff would still use ActiveX, Flash, Java, or just live outside the browser altogether (Napster...). Then a combination of JS improvements (XHR), publicity efforts (Crockford and GMail), and the market standardizing on minimum common denominator (avoiding deployment and security issues of plugins) left us alone in the room with this beast.