So, what is Zepto.js?

Ian Pointer

October 2013

(For more resources related to this topic, see here.)

One of the most influential JavaScript libraries in the last decade of web development is jQuery, a comprehensive set of functions that make Document Object Model (DOM) selection and manipulation consistent across a range of browsers, freeing web developers from having to handle all these themselves, as well as providing a friendlier interface to the DOM itself.

Zepto.js is self-described as an aerogel framework—a JavaScript library that attempts to offer the most of the features as the jQuery API, yet only taking up a fraction of the size (9k versus 93k in the default, compressed current versions Zepto.js v1.01 and jQuery v1.10 respectively). In addition, Zepto.js has a modular assembly, so you can make it even smaller if you don't need the functionality of extra modules. Even the new, streamlined jQuery 2.0 weighs in at a heavyweight 84k.

But why does this matter?

At a first glance, the difference between the two libraries seems slight, especially in today's world where large files are normally described in terms of gigabytes and terabytes. Well, there are two good reasons why you'd prefer a smaller file size. Firstly, even the newest mobile devices on the market today have slower connections than you'll find on most desktop machines. Also, due to the constrained memory requirements on smartphones, mobile phone browsers tend to have limited caching compared to their bigger desktop cousins, so a smaller helper library means more chance of keeping your actual JavaScript code in the cache and thus preventing your app from slowing down on the device. Secondly, a smaller library helps in response time—although 90k versus 8k doesn't sound like a huge difference, it means fewer network packets; as your application code that relies on the library can't execute until the library's code is loaded, using the smaller library can shave off precious milliseconds in that ever-so-important time to first-page-load time, and will make your web page or application seem more responsive to users.

Having said all that, there are a few downsides on using Zepto.js that you should be aware about before deciding to plump for it instead of jQuery. Most importantly, Zepto.js currently makes no attempt to support Internet Explorer. Its origins as a library to replace jQuery on mobile phones meant that it mainly targeted WebKit browsers, primarily iOS. As the library has got more mature, it has expanded to cover Firefox, but general IE support is unlikely to happen (at the time of writing, there is a patch waiting to go into the main trunk that would enable support for IE10 and up, but anything lower than Version 10 is probably never going to be supported). In this guide we'll show you how to include jQuery as a fallback in case a user is running on an older, unsupported browser if you do decide to use Zepto.js on browsers that it supports and want to maintain some compatibility with Internet Explorer.

The other pitfall that you need to be aware of is that Zepto.js only claims to be a jQuery-like library, not a 100 percent compatible version. In the majority of web application development, this won't be an issue, but when it comes to integrating plugins and operating at the margins of the libraries, there will be some differences that you will need to know to prevent possible errors and confusions, and we'll be showing you some of them later in this guide.

In terms of performance, Zepto.js is a little slower than jQuery, though this varies by browser (take a look at to see the latest benchmark results). In general, it can be up to twice as slow for repeated operations such as finding elements by class name or ID. However, on mobile devices, this is still around 50,000 operations per second. If you really require high-performance from your mobile site, then you need to examine whether you can use raw JavaScript instead—the JavaScript function getElementsByClassName() is almost one hundred times faster than Zepto.js and jQuery in the preceding benchmark.

Writing plugins

Eventually, you'll want to make your own plugins. As you can imagine, they're fairly similar in construction to jQuery plugins (so they can be compatible). But what can you do with them? Well, consider them as a macro system for Zepto.js; you can do anything that you'd do in normal Zepto.js operations, but they get added to the library's namespace so you can reuse them in other applications.

Here is a plugin that will take a Zepto.js collection and turn all the text in it to Helvetica font-family at a user-supplied font-size (in pixels for this example).

(function($){ $.extend($.fn, { helveticaize: function( options ){ $.each(this, function(){ $(this).css({"font-family":"Helvetica", "font-size": options
['size']+'px'}); }); return this; } }) })(Zepto || jQuery)

Then, to make all links on a page Helvetica, you can call $("a").helveticaize(). The most important part of this code is the use of the $.extend method. This adds the helveticaize property/function to the $.fn object, which contains all of the functions that Zepto.js provides.

Note that you could potentially use this to redefine methods such as find(), animate(), or any other function you've seen so far. As you can imagine, this is not recommended—if you need different functionality, call $.extend and create a new function with a name like custom_find instead. In addition, you could pass multiple new functions to $.fn with a call to $.extend, but the convention for jQuery and Zepto.js is that you only provide as few functions as possible (ideally one) and offer different functionality through passed parameters (that is, through options). The reason for this is that your plugin may have to live alongside many other plugins, all of which share the same namespace in $.fn. By only setting one property, you hopefully reduce the chance of overriding a method that another plugin has defined.

In the actual definition of the method that's being added, it iterates through the objects in the collection, setting the font and size (if present) for all the objects in the collection. But at the rest of the method it returns this. Why? Well, if you remember, part of the power of Zepto.js is that methods are chainable, allowing you to build up complex selectors and operations in one line. And thanks to helveticaize() returning this (which will be a collection), this newly-defined method is just as chainable as all the default methods provided. This isn't a requirement of plugin methods but, where possible, you should make your plugin methods return a collection of some sort to prevent breaking a chain (and if you can't, for some reason, make sure to spell that out in your plugin's documentation).

Finally, at the end, the (Zepto || jQuery) part will immediately invoke this definition on either the Zepto object or jQuery object. In this way, you can create plugins that work with either framework depending on whether they're present, with the caveat, of course, that your method must work in both frameworks.


In this article, we learned what Zepto.js actually is, what you can do with it, and why it's so great. We also learned how to extend Zepto.js with plugins.

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