I use UglifyJS to compress and mangle my JS and stick it in a bookmarklet.

This is the bookmarklet's JS (the anon function is to make sure it evaluates to undefined):

(function() {
  /*bookmarklet stuff*/
})();

This is the JS after compression (notice the exclamation point):

!function(){/*bookmarklet stuff*/}();

The exclamation point negates the return value of the anon function so instead of evaluating to undefined, it will evaluate to true. Then when I prepend the javascript: "protocol" the bookmarklet instead navigates to a blank page displaying true.

IMO UglifyJS shouldn't negate anonymous functions if the return value is used or in this case if the function is the root function.