D8 Mobile Initiative IRC Meeting #2

Lazy loading is a very small subset of asynchronous resource loading. We shouldn't confuse the two terms as we make plans for Drupal's future.

In one scenario, scripts are loaded by JavaScript itself (yepnope.js, LABjs and others) to avoid blocking the rendering of the web page. Although it performs additional HTTP requests, when configured properly it often speeds up the rendering of a web page on the desktop. There is still debate as to whether or not blindly using async script loading is beneficial on mobile (think LABjs), but combined with conditional async loading, you almost always achieve gains by the very fact that you download only what you need. This is why yepnope.js and Modernizr are such a great pair: Modernizr has all the conditionals, yepnope does the loading.

Lazy loading, however, normally involves delaying the loading of assets until the page thinks they're needed. Most often we see this in the form of lazy-loaded images on a high-traffic blog. We do not want to bake lazy-loading into Drupal core. I imagine it would be a delightful feature for displaying long lists of media, like in Views.

There is another option we have available to us, which is deferred scripts. In some cases, you can set an HTML attribute defer on your script tags directly, which tells the browser to simply download the script and wait until the page has finished fetching other assets before blocking the page render and executing the script. This is precarious for historical reasons, which require the developer to essentially make a "pledge" (which can be broken with no warning) to write code that never uses the features of JavaScript that block rendering (document.write() is an example of a function you cannot use with defer).

Finally, there are rumblings of introducing a similar async attribute to script tags, which would no only defer, but also execute asynchronously. There is no "developer pledge" required in this case because it's always async so those rules apply. It would not alleviate the need for JS-based async script loaders because they are the only means to conditional loading at this time, which is what mobile is all about.