Javascript is invented by Brendan Eich who founded Mozilla and was briefly the CTO of Mozilla until he was fired because he doesn't support gay marriage. Although an old web technology but together with HLTML 5, they will define web technology of the future and may make flash redundant. One of the big plus of Javascript is that it is client-side or browser dependent technology.
You can also make use of toolkits and libraries like JQuery and Dojo. There is also a mobile version like jQuery Mobile and PhoneGap
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