How can I write this lambda closure in CoffeeScript?

do ($, window) ->
  $ ->
    alert "js!"

compiles to

(function($, window) {
  return $(function() {
    return alert("js!");
  });
})($, window);

undefined is a keyword in CoffeeScript. You don't need to ensure it's properly defined, so you can forget that part.

CoffeeScript provides a do keyword that you can use to create a closure instead of using the immediately-invoked function expression syntax.

CoffeeScript Source ^{try it}

do ($ = jQuery, window) ->  
  $ ->  
    alert "js!"

Compiled JavaScript

(function($, window) {
  return $(function() {
    return console.log("js!");
  });
})(jQuery, window);

The above syntax wasn't supported until CoffeeScript 1.3.1. For older version you still need to do this:

CoffeeScript Source ^{[try it]}

(($, window) ->
  $ ->
    alert "js!"
)(jQuery, window)

If you're curious, here's how CoffeeScript handles undefined.

CoffeeScript Source ^{[try it]}

console.log undefined

Compiled JavaScript

console.log(void 0);

You can see that it doesn't use the undefined variable, but instead uses JavaScript's void operator to produce the undefined value.

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