Note: LazyLoad is no longer being maintained by its original author, Ryan Grove; he's not responding to issues or pull requests, since I doesn't use this project anymore and doesn't have time to work on it. Please feel free to fork it if you want to continue development on it.

LazyLoad is a tiny (about 1KB minified and gzipped), dependency-free JavaScript utility that makes it super easy to load external JavaScript and CSS files on demand.

Whenever possible, LazyLoad will automatically load resources in parallel while ensuring execution order when you specify an array of URLs to load. In browsers that don't preserve the execution order of asynchronously-loaded scripts, LazyLoad will safely load the scripts sequentially.

Use LazyLoad when you need a small, fast, safe dynamic JS or CSS loader, but don't need the overhead of dependency management or other extra functionality that larger script loaders provide.

• lazyload.js (full source)

Using LazyLoad is simple. Just call the appropriate method -- css() to load CSS, js() to load JavaScript -- and pass in a URL or array of URLs to load. You can also provide a callback function if you'd like to be notified when the resources have finished loading, as well as an argument to pass to the callback and a context in which to execute the callback.

// Load a single JavaScript file and execute a callback when it finishes.
LazyLoad.js(['http://example.com/foo.js'], function () {
  alert('foo.js has been loaded');
});

// Load multiple JS files and execute a callback when they've all finished.
LazyLoad.js(['foo.js', 'bar.js', 'baz.js'], function () {
  alert('all files have been loaded');
});

// Load a CSS file and pass an argument to the callback function.
LazyLoad.css(['foo.css'], function (arg) {
  alert(arg);
}.bind(null, 'foo.css has been loaded'));

// Load a CSS file and execute the callback in a different scope.
LazyLoad.css('foo.css', function () {
  alert(this.foo); // displays 'bar'
}.bind({foo: 'bar'}));

• Firefox 3.5+
• Google Chrome 10+
• Internet Explorer 9+
• Opera 9+
• Safari 3+
• Mobile Safari
• Android

Other browsers may work, but haven't been tested. It's a safe bet that anything based on a recent version of Gecko or WebKit will probably work.

I have attempted to restore support for the browsers that the original LazyLoad works on:

• Firefox 2+
• Google Chrome 0.8+
• Internet Explorer 6+
• Opera 9+

All browsers support parallel loading of CSS. However, only Firefox and Opera as of 2011 supported parallel script loading while preserving execution order. To ensure that scripts are always executed in the correct order, LazyLoad will load all scripts sequentially in browsers other than Firefox and Opera. Hopefully other browsers will improve their parallel script loading behavior soon.

Copyright (c) 2011 Ryan Grove (ryan@wonko.com). All rights reserved. Substantial modifications made by Cezary Daniel Nowak (2014) and James Edward Lewis II (2015).

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.