A JavaScript library to load images progressively 🌇
JavaScript CSS



Progressively is a javascript library for loading images progressively. It’s written entirely in JavaScript so it doesn’t depend on 3rd-party libraries like jQuery. It's super small, < 1.2kB when minified & gzipped! It will load the full-size images only when the user browses to that part of the page, saving bandwidth & server requests. It is compatible with all modern browsers. See the Demo.


Usage & Api


bower install progressively
npm install progressively
  <link src="progressively.min.css">

  <figure class="progressive">
    <img class="progressive__img progressive--not-loaded" data-progressive="img/highQualityImg" src="img/lowQualityImg">

  <script src="progressively.min.js"></script>

The src attribute will contain the lower quality image (< 20kb for ideal cases) and data-progressive attribute will hold the path/url to high quality image. See demo for examples.

.init() (options)

The init() API has a few options


Type: Number Default: 300

The throttle is managed by an internal function that prevents performance issues from continuous firing of window.onscroll events. Using a throttle will set a small timeout when the user scrolls and will keep throttling until the user stops. The default is 300 milliseconds.


Type: Number Default: 100 value

The delay function sets the timout value for images to start load asynchronously. Ideally it's value should be low.


Type: Function Arguments: None

The afterload function is callback function which executes when all images have loaded. It is fired when all the image elements have the *--is-loaded class.


Type: Function Arguments: HTMLElement

The imgload function is invoked whenever an image elements finishes loading. It accepts HTMLElement as an argument which is the current element that is loaded.

  delay: 50,
  throttle: 300,
  imgload: function(elem) {
  afterload: function() {
    console.log('All images have finished loading!');


Progressively has a render() method that can be used to make progressively poll your images when you're not scrolling. For instance in some case you want to render your images before/widthout scrolling down to the image, you can use render


Make sure you follow linting guidelines. For bugs or feature request, open an issue. Create a seprate branch by the name of your feature request and then send PR.


MIT license