These docs are for the developer preview of Mobify.js 2.0.

Looking for the 1.1 documentation? It's right here!

Quick Start

What is Mobify.js?

Mobify.js is an open source library for improving responsive sites by providing responsive images, JS/CSS optimization, Adaptive Templating and more. Mobify.js also provides a “Capturing” API for manipulating the DOM before any resources have loaded, giving developers the ability to enable the listed features above without changing any backend markup.

Getting started

Let’s get started by using the Image API (one of many APIs available in mobify.js) to automatically resize and optimize images in your page.

  • Note: If you’re using this API locally and your images aren’t publicly available, the original images will load. To see the images resize, try this on an environment that is publicly available.
With Capturing

If you don’t want to have to worry about changing src attributes, you can let Capturing take care of that for you. It requires a special script tag that must be placed after the opening <head> tag (must be placed above any element that loads external resources!): Unminified version on Github

If you have purchased a Mobify platform subscription, please head over to to get your tag instead.

<script>!function(a,b,c,d,e){function g(a,c,d,e){var f=b.getElementsByTagName("script")[0];a.src=e,,a.setAttribute("class",d),f.parentNode.insertBefore(a,f)}a.Mobify={points:[+new Date]};var f=/((; )|#|&|^)mobify=(\d)/.exec(location.hash+"; "+b.cookie);if(f&&f[3]){if(!+f[3])return}else if(!c())return;b.write('<plaintext style="display:none">'),setTimeout(function(){var c=a.Mobify=a.Mobify||{};c.capturing=!0;var f=b.createElement("script"),h="mobify",i=function(){var c=new Date;c.setTime(c.getTime()+3e5),b.cookie="mobify=0; expires="+c.toGMTString()+"; path=/",a.location=a.location.href};f.onload=function(){if(e)if("string"==typeof e){var c=b.createElement("script");c.onerror=i,g(c,"main-executable",h,mainUrl)}else a.Mobify.mainExecutable=e.toString(),e()},f.onerror=i,g(f,"mobify-js",h,d)})}(window,document,function(){a=/webkit|(firefox)[\/\s](\d+)|(opera)[\s\S]*version[\/\s](\d+)|(trident)[\/\s](\d+)/i.exec(navigator.userAgent);return!a||a[1]&&4>+a[2]||a[3]&&11>+a[4]||a[5]&&6>+a[6]?!1:!0},

// path to mobify.js

// calls to APIs go here (or path to a main.js)
function() {
  var capturing = window.Mobify && window.Mobify.capturing || false;

  if (capturing) {
      var capturedDoc = capture.capturedDoc;

      var images = capturedDoc.querySelectorAll("img, picture");
      // Render source DOM to document
Without Capturing

If you want to use the Image API without Capturing, you must change src to x-src (this is configurable) for every <img> and <picture> element you have in your site (you also may want to add <noscript> fallbacks if you’re worried about browsers with JavaScript disabled/unavailable). This snippet will load mobify.js asynchronously in order to be able to start loading images before the DOM is ready.

Then, paste the following tag before </head>, or top of <body>:

If you have purchased a Mobify platform subscription, please head over to to get your tag instead.

<script async src="//"></script>
    var intervalId = setInterval(function(){
        if (window.Mobify) {
            var images = document.querySelectorAll('img[x-src], picture');
            if (images.length > 0) {
            // When the document has finished loading, stop checking for new images
            if (Mobify.Utils.domIsReady()) {
    }, 100);

Where to next?