I think the hardest part of JavaScript or any language for that matter is learning what is possible. For just the absolute basics, if you're already familiar with programming in general, 7 days seems possible. If you are unfamiliar with programming concepts in general like loops and events and variables then learning JS in 7 days seems unlikely.
For a JS frontend, if you are unfamiliar with the HTML DOM and unfamiliar with CSS then no, 7 days isn't nearly enough for even the basics. If you are already familiar with HTML & CSS then I would say 7 days is possible for a very basic understanding of JS.
For a JS backend, if you are unfamiliar with programming concepts or databases, then 7 days is also unlikely to be enough.