Javascript Defer (Async vs Defer)

<script type="text/javascript">function appReady() {    // do something}</script><script defer src="/js/app.b5799d78449381e12b5d.js" onload="appReady()"></script>

When you use PageSpeed Insights, it shall complaint of Remove render-blocking JavaScript.

One of the way to solve this is using defer/async loading of JavaScript.

Defer vs Async

Refer to Script Tag - async & defer.

Defer

• Guarantee order of exectution
• Execute after HTML parser is completed

Async

• No guarantee order of execution (suitable for js like Google Analytics)
• Pause HTML parser to execute when finish downloading