It's tough to say the exact cause of this issue, but if you don't have a javascript.vim file you probably should. I suggest starting with this enhanced javascript syntax config. It is likely to fix your javascript folding issue, and much more.

If you just want to focus on the folding issue you might try creating your own javascript.vim file in ~/.vim/syntax/javascript.vim that contains code along the lines of what I have given below. You may want to adjust the fold level to your liking (0 is completely folded). However, this simple version will not play well with comments containing curly bracket characters, which is where you will want to go with a more robust javascript.vim like the one I have linked.

syntax region foldBraces start=/{/ end=/}/ transparent fold keepend extend
setlocal foldmethod=syntax
setlocal foldlevel=0

I should add that both myself and the other responder are suggesting that you need a javascript.vim, and in fact by some of the same contributors. However, the one I am suggesting was last updated in December of 2015 as opposed to 2009.

I don't know why your solution isn't working, but a possible solution is to use a user-created vimscript available at http://www.vim.org/scripts/script.php?script_id=1491

Just had this same issue answered on Vim Stack Exchange, and the answer is that if you do use the stock syntax/javascript.vim file, then you have to set

    vv
let g:javaScript_fold = 1
    ^^

The difference between the command in the question and here is the g: part (highlighted above). I'm new to Vim scripting, but I believe the difference is that let javaScript_fold=1 sets a script-local variable, making it confined to your .vimrc file, and the example above makes it global (which seems to be confirmed by this Stackoverflow thread). See more on this at section 41.2 Variables in :help usr_41.txt and :help internal-variables.

This Reddit thread was also enlightening; it's not JavaScript-related but the folding seems to be useful for JS files as well.