README.md

Example output, where | is the cursor:

function () {
  |
};

foo(function () {
  |
});

if () {
  |
}

Along with else, for, and switch.

Also does this correctly:

var x = {
  foo: function () {
    |
  } // No semi!
}; // Semi!

js-fu.vim

fun! SemiOrNoSemi()
    return "{\<cr>}\<c-o>O\<Tab>\<Down>".(getline('.') =~# '^\s*for\s\|if\s\|else\s|switch\s'||getline('.') =~# ':'? '' : ';')."\<up>"
endfunction

au BufNewFile,BufRead *.js imap <buffer> <expr> {<cr> SemiOrNoSemi()