// Tutorial //

Eliminate Render-Blocking JavaScript with Async and Defer

Published on August 20, 2016
    Default avatar

    By Alligator.io

    Eliminate Render-Blocking JavaScript with Async and Defer

    While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

    With HTML5, we get two new boolean attributes for the <script> tag: async and defer. Async allows execution of scripts asynchronously and defer allows execution only after the whole document has been parsed.

    These two attributes are a must for increasing speed and performance of websites. They allow the elimination of render-blocking JavaScript where the page would have to load and execute scripts before finishing to render the page. Here’s a usage example:

    <script defer src="/js/jquery.min.js">

    Async vs Defer

    With async, the file gets downloaded asynchronously and then executed as soon as it’s downloaded.

    With defer, the file gets downloaded asynchronously, but executed only when the document parsing is completed. With defer, scripts will execute in the same order as they are called. This makes defer the attribute of choice when a script depends on another script. For example, if you’re using jQuery as well as other scripts that depend on it, you’d use defer on them (jQuery included), making sure to call jQuery before the dependent scripts.

    A good strategy is to use async when possible, and then defer when async isn’t an option.

    👉Note that both attributes don’t have any effect on inline scripts.

    Browser Support

    Can I Use script-async? Data on support for the script-async feature across the major browsers from caniuse.com.

    Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

    Learn more about us

    Want to learn more? Join the DigitalOcean Community!

    Join our DigitalOcean community of over a million developers for free! Get help and share knowledge in our Questions & Answers section, find tutorials and tools that will help you grow as a developer and scale your project or business, and subscribe to topics of interest.

    Sign up now
    About the authors
    Default avatar


    Still looking for an answer?

    Ask a questionSearch for more help

    Was this helpful?
    Leave a comment...

    This textbox defaults to using Markdown to format your answer.

    You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!

    This comment has been deleted

    Try DigitalOcean for free

    Click here to sign up and get $200 of credit to try our products over 60 days!
    Try DigitalOcean for free