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lazyload.js

README.md

LazyLoad

LazyLoad is a tiny (only 904 bytes minified and gzipped), dependency-free JavaScript library that makes it super easy to load external JavaScript and CSS files on demand. It's ideal for quickly and unobtrusively loading large external scripts and stylesheets either lazily after the rest of the page has finished loading or on demand as needed.

In addition to CSS support, this version of LazyLoad also adds support for parallel loading of multiple resources in browsers that support it. To load multiple resources in parallel, simply pass an array of URLs in a single LazyLoad call.

Downloads

Usage

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);
}, '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'
}, null, {foo: 'bar'});

Supported Browsers

  • Firefox 2+
  • Google Chrome (all versions)
  • Internet Explorer 6+
  • Opera 9+
  • Safari 3+
  • Mobile Safari (all versions)

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.

Caveats

All browsers support parallel loading of CSS. However, only Firefox and Opera currently support 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.

Sadly, Firefox doesn't provide any indication when a CSS file has finished loading. In Firefox, CSS load callbacks will execute after a short delay, but there's no way to automatically guarantee that the CSS has finished loading before the callback is executed. Luckily, there's a fairly painless manual workaround that you can use to detect when CSS has finished loading, but it's not possible for LazyLoad to do it for you.

License

Copyright (c) 2011 Ryan Grove (ryan@wonko.com). All rights reserved.

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.

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