Modifying Native JavaScript Objects

Random Number Generator There are still debates in JavaScript community about modifying native JavaScript objects (Array, Number, String, Object). Some developers believe it’s evil, while other encourage it. If you’ve looked at RapydScript’s stdlib, you probably already know my stance on it – I’m actually in favor of such modifications when they make sense. I might not have as much experience in JavaScript as the gurus, but I have enough experience in language design to form sane opinions about which practices are evil and which are not.

Most arguments against modifying the native objects hold no water. They give examples where the developer overrides a native method to work differently from original implementation. That is indeed evil, assuming that calling this method with the same arguments as original JavaScript implementation no longer produces the same result. For example, if I decided to rewrite String.prototype.replace such that it worked globally, like in most other languages, instead of on a first occurrence:

String.prototype.replace = function(orig, sub) {
    return this.split(orig).join(sub);

I could break (or make them behave differently than they should) other libraries/widgets on the same page that assume that replace() will only replace the first occurrence. I completely agree that one should never override existing methods to do something else. It’s important that the basic subset of the JavaScript language works exactly as others would expect it to. If you can’t guarantee that the foundation of your house stays level, you can’t guarantee that your house will not collapse.

There are some gray area cases, where the developer could extend the functionality of existing method, such that it still works as expected given original arguments, but does something else when more arguments are given:

Array.prototype.pop = function(index) {
    if (!arguments.length) {
        index = this.length - 1;
    return this.splice(index, 1)[0];

In this case, myArray.pop() will still work exactly as the user would expect. However, if called with myArray.pop(0), the function will behave like myArray.shift(), or like splice() method if called with an index in the middle. The only real disadvantage here is that by overriding the native method, the developer has made the function slower than the original (native methods tend to work faster). Claiming that this is bad because a function could break another logic that mistakenly called it with an argument (which it ignored before) on the other hand is not a legitimate argument. The bug is in the function making the bad call, not in the overriding logic. This is why I like Python, it will complain about unexpected arguments right away instead of ignoring them so they could become a bigger problem later.

Appending your own methods, on the other hand, is the most benign way to modify a native object. This is when we implement something like this, for example:

Array.prototype.copy = function() {
    return this.slice();

When doing, so, however, it’s important to be aware if other libraries are appending a method with the same name but different functionality, then you might want to pick a different name (this is usually not a problem with libraries/APIs since you’re unlikely to need more than one library for native object manipulation). So far, the most popular argument I’ve seen against this type of native object modification is potential name collision if future version of JavaScript decides to add a method by the same name. This argument is moot. You’re not developing your app for a hypothetical language that will exist 10 years from now, and when the time comes you’ll easily be able to rename the offending function, since you know all references to it are your own. The entire EcmaScript specification is easily available, and unless you plan to drop support for all browsers older than 6 months, the JavaScript implementation you’ll be using as basis will probably lag behind that by a few months to a few years, giving you more than enough time to handle naming collisions.

You might notice, however, that if you start appending methods to the native Object object, any jQuery code running on the same page will break. The problem is not with overwriting native methods, but with poor assumptions made by jQuery developers who wrote buggy code. The problem is that jQuery developers are part of the group that believe that appending to native JavaScript objects is evil, and instead of properly testing that they’re iterating through object’s own properties, they make the assumption that no one else will append anything to Object when they scan through it using “for key in Object” type logic. To avoid the same mistake as jQuery made, make sure to iterate only through your own keys:

for (var key in obj) {
    if obj.hasOwnProperty(key) {

Alternatively, if you only care about the latest browsers, you can use Object.getOwnPropertyNames() instead. It was a poor design on JavaScript’s part to default to iterating through every property of the object, but that can’t be changed now. jQuery developers have been confronted about their assumption before, they claim the main reason was performance. Independent tests showed about 5% performance hit from adding this check, so I don’t buy the performance claim, especially since jQuery already makes multiple performance sacrifices in the name of usability (show/hide logic having safety checks to figure out the object’s current state, for example). In my opinion, John Resig’s stance on this is no different than failing to do a “division by zero” check (in Python, because JavaScript will automatically return infinity) and then claiming that it’s for performance reasons and anyone who passes arguments that eventually result in a division by zero is the one at fault.

I haven’t checked if this is fixed in jQuery 2.0. Without support for older browsers there is no reason not to use Object.getOwnPropertyNames(). In the meantime, try not to overwrite Object (other native objects are fine) if you’re using jQuery anywhere at all (if you’re not, it’s not a problem). I should also mention that if you don’t plan to support older browsers (such as IE8), a better practice would be to use defineProperty method rather than appending to the prototype, since it will omit the method from getting picked up when iterating through the object, making the jQuery bug a non-issue.

As far as my own stance, it’s fine to append to the functionality of a native object, but never to remove. Modifying existing property to work differently than before, given the same function signature, counts as removal, and is evil as well. I have removed the dependence on overwriting native methods in RapydScript’s stdlib2, but that was mainly to clean up the standard library and make it compatible with jQuery, not because I’m against modifying native objects.

This entry was posted in Languages and tagged , , by Alexander Tsepkov. Bookmark the permalink.

About Alexander Tsepkov

Founder and CEO of Pyjeon. He started out with C++, but switched to Python as his main programming language due to its clean syntax and productivity. He often uses other languages for his work as well, such as JavaScript, Perl, and RapydScript. His posts tend to cover user experience, design considerations, languages, web development, Linux environment, as well as challenges of running a start-up.

4 thoughts on “Modifying Native JavaScript Objects

  1. How does rapydscript relate to asm.js? Wxpython now works with python 3 – can rapydscript make use of it?

    • I’m not familiar with asm.js, so to answer your question, they don’t relate. Reading about asm.js, it seems like it’s a subset of JavaScript, but since that wasn’t the use case I targeted for RapydScript, I couldn’t tell you if the code generated from RapydScript would be compatible with asm.js or not.

      As far as wxPython + Python 3, that’s a completely different language/environment combination. RapydScript is still JavaScript, but with abstractions that make it intuitive to Python developers, RapydScript is designed to work in a browser. RapydScript can also run on a Desktop through something like node.js, but it would not be able to interface with wxPython, although it would be able to interface with wxJavaScript. wxPython, on the other hand, is a C++/Python library that has no chance of running in the browser and couldn’t interface with JavaScript.

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