Image updater for responsive designs


molt 4.2.2

This package is now marked as obsolete, please use picturefill instead

Molt is a tiny script that updates images according to the current viewport's width or height to have a better responsive design. It's built in top of W.


You can pick the minified library or install it with :

bower install molt
npm install pyrsmk-molt --save-dev
jam install pyrsmk-molt

What's new since 3.0

  • supports viewport's width and height
  • two different syntaxes
  • handle callbacks with promises to know when images are loaded
  • the hability to hide an image is let to the user


The library basic version you want to have is the minimized one with W inside, the so called molt.W.min.js. But if you have planned to use W standalone or from another library, you can pick the molt.min.js one.


A molt image is set as a regular img tag on your HTML body with some attributes that tell when to load such or such image, but no src attribute which will be set by molt itself. Images are loaded by modes which are similar to media queries. They are interpreted like a media query's min-width or min-height condition. Per example, the 728w mode will load the corresponding image when the viewport has a minimum width of 728px. Likewise, a 480h will load the image if the viewport's height is at least 480px. So, let's see how we need to write our markup :

<img data-molt-320w="images/image320w.jpg" data-molt-768w="images/image768w.jpg" data-molt-1600w="images/image1600w.jpg">

Pretty simple, isn't it? But that syntax could be really verbose on serveral circumstances. Here's an alternative that is doing the same by replacing the {} tag in the url by the current mode :

<img data-molt-src="images/image{320w,768w,1600w}.jpg">

If, for any reason, you cannot rename you image files by including its corresponding mode, you can specify something more intelligible :

<img data-molt-src="images/some_wonderful_picture-{320w:tiny,768w:medium,1600w:large}.jpg">

Then, if the current mode is 728w the images/some_wonderful_picture-medium.jpg image will be loaded. Please note that if no mode is matched (when the user's viewport is less than the minimal mode your set) the image won't show up.

Now we have set our img tags, we can start the engine with the node list of the images you want molt to handle :



Molt is working by using something called promises. They are chainable methods that register callbacks. Those promises will let us handle image loadings and start all the engine when we're ready, and you can use any promise any times you want. But examples are better than words :

        // Called before downloading new images
        // Called for each loaded image but once (useful for initializations)
        // Called for each loaded image
        // Called when all images have been loaded but once (useful for initializations)
        // Called when all images have been loaded

Some parameters are passed to the callbacks the mode (could be 768w, 800h or anything else), and the image's node for each() or the node list for then().


Currently, if you want to use several instances of molt, all your images will be returned into the promises of each instance.


Molt is released under the MIT license.