Haskell is pure so will have no other choice to force you to express your algorithms in a functional way. If you have time to learn Haskell, and force you to program non trivial algorithms with it, then you will have a really good understanding of functional programming. Even if you don't want to use it, just looking at the overview part and seeing how CoffeeScript translates to JavaScript will make you understand better JavaScript. This language is basically JavaScript with a better syntax, and it brings out the functional side of JavaScript. You might also be interested in CoffeeScript which is 'a little language that compiles into JavaScript'. By the way, you should take a look at Underscore, which is a library for JavaScript that brings a lot of functions such as map.

I will add moreover that JavaScript hides a bit its functional programming and does not naturally provide functions such as map, reduce, composition, filter and functional characteristics such as currification. So each time you have difficulty writing something in a functional way, you will probably abandon and switch to your old habit. You could learn functional programming using JavaScript, but the fact that JavaScript is not purely functional and allows you to write easily imperative code, won't force you to write difficult algorithms in a functional way. But learning Haskell will make you understand functional programming better in my opinion. Honestly, you don't have to learn Haskell to understand functional programming. I think there is no binary answer to your question.

Learn haskell functional programming