defer VS async

引用

The defer Attribute

The defer attribute makes a solemn promise to the browser. It states that your JavaScript does not contain any document.write or DOM modification nastiness:

<script src="file.js" defer></script>

The browser will begin to download file.js and other deferred scripts in parallel without stopping page processing.defer was implemented in Internet Explorer version 4.0 — over 12 years ago! It’s also been available in Firefox since version 3.5.While all deferred scripts are guaranteed to run in sequence, it’s difficult to determine when that will occur. In theory, it should happen after the DOM has completely loaded, shortly before the DOMContentLoaded event. In practice, it depends on the OS and browser, whether the script is cached, and what other scripts are doing at the time.
The async Attribute

async has been introduced in HTML5:

<script src="file.js" async></script>

async is identical to defer, except that the script executes at the first opportunity after download (an optional onload attribute can be added to run a specific function). You can’t guarantee that scripts will execute in sequence, but they will have loaded by the time the window onload event fires.There’s support for async in Firefox 3.6, Opera 10.5, and the latest WebKit build, so it should appear in the next versions of Chrome and Safari. IE9 is yet to support async, but the IE team could easily add it as an alias for defer. You can use both async and defer to support all browsers — even IE4.Perhaps within a few months, we’ll finally have a native, non-blocking JavaScript loading method that works in all browsers.

https://developer.mozilla.org/en-US/docs/HTML/Element/script