[mtphil]

I'd just like to add some votes for resources already mentioned:

MDN's documentation is by far the best. A very easy entry would be to use Codecademy, while more in depth video tutorials can be found on Pluralsight and Egghead (though Pluralsight also has some introductory and rapid training courses focused on Javascript). The best book for learning how JS works under the surface is Simpson's series 'You Don't Know JS'.