The Onist

The Onist


Monkey Patching in JavaScript

Learn how to "monkey patch" or modify the runtime Javascript without altering the original source code.


Monkey Patching is technique to "extend or modify the runtime code of dynamic languages without altering the original source code". Wikipedia then goes on to outline four common used for monkey patching:

Patching a Third-Party Library

After working for years on a very large web platform, I have quickly learned one important lesson: never directly modify the source of a third-party library! Doing so makes upgrading the library a total nightmare! Read this blog post for a more detailed explanation. So what do you do if you're waiting for the library creators to either A) add your needed feature or B) fix a bug? Why you monkey patch of course!

In the following example, I'm monkey patching jQuery's .appendTo() function to alternate a pages background color based on the amount of children elements on document.body.

// create a closure and remap jQuery to $
	// save off original method
	var _originalAppendTo = $.fn.appendTo;
  	// override method
	$.fn.appendTo = function() {
	  // silly code to alternate backgound color
	if ($(document.body).children().length % 2) { = "#f285cf";
	} else { = "#85C3F2";

	// call original passing in scope and arguments
	return _originalAppendTo.apply(this, arguments);

Dissecting the above code, first we create a closure around the patch to remap jQuery to $. This also gives us the ability to use local variables if need be. Next, we store a reference to the original function before we mask it. This gives us the ability to call the original at any point in our masking function. After this, we then declare the masking function on top of the originals namespace ($.fn.appendTo). Finally, we implement our patch and are able to call back to the original at anytime. Sweet!

Here is the full example implemented on JsFiddle:

Potential Problems

As with any programming construct, monkey patching has both benefits and pitfalls. I have just shown you a few examples of the benefits. Now here are some of the potential problems:

With all this being said, all uses of the monkey patching pattern should be heavily scrutinized and implemented with caution as it can lead to confusion and bugs on larger projects. Now get out there and...

monkey patch all the things

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