Question

Here is a common practice in JavaScript:

(function($) {
    ...code...
})(jQuery);

I understand the wrapper function (it prevents pollution of the global namespace), but many libraries (like jQuery, Underscore, etc.) already define short names ($ and _, respectively) at global scope for me to use. I wonder what the advantage to this approach is. Just to rename jQuery to something shorter? Prevent me from overwriting $? Make it easier to swap in another library later? I guess none of these seem really convincing to me.

Furthermore, I have also seen this:

(function(_) {
    ...code...
})(_);

Nothing is even renamed here. I have even seen:

(function(global) {
    ...code...
})(this); // or window, perhaps

What is wrong with just using window directly?

So here’s what I’m asking:

• Does this practice have a name?
• What are the advantages to this practice?
• Should I always pass in libraries I’m using rather than use them directly?
• Should I pass in this or window as a reference to global scope?

Answer 1

Does this practice have a name?

The syntax is referred to as a self-executing anonymous function. I’m not aware of any special name for passing global objects to the function.

What are the advantages to this practice?

• As you’ve noted, var scoping variables within the function will
  help de-clutter the global namepace.

• The jQuery example is typically used in plugins so that the plugin can make use of $ while remaining compatible with $.noConflict() (docs)

  (function($) {
      ...code...
  })(jQuery);
  

• In general, passing global or reserved objects (like window, document) as parameters can help reduce the size of your script after minification:

  (function(window, document) {
      // A JS minifier can now minify the all occurrences of `window` and
      // `document` within this function.
  })(window, document); 
  

Should I always pass in libraries I’m using rather than use them directly?
Should I pass in this or window as a reference to global scope?

Only if minification or conflicting library variable names are a concern.