How to Defer Parsing of JavaScript in WordPress
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How to Defer Parsing of JavaScript in WordPress

If you want to cut your website’s loading time, you can try to defer parsing of JavaScript. It allows the browser to render JavaScript only after it finishes loading the main content of a site. In this tutorial, we will cover several ways on how to defer parsing of JavaScript in WordPress.

Why Should You Defer Parsing of JavaScript?

When you open a website, your browser will receive the site’s content from the server and loads the code from top to bottom. However, if it finds JavaScript, the loading process will be interrupted until it finishes downloading all the JavaScript.

To solve this issue, you can defer parsing of JavaScript, which allows the browser to load the full content without waiting for the scripts to load. By implementing this task, JavaScript parsing won’t negatively affect your website’s loading time.

Keep in mind that having lots of JavaScript isn’t bad. After all, It’s one of the essential web components, which allows you to extend the functionality of your site.

The point is, there are two main benefits you can get:

  • Better user experience – visitors won’t have to wait long to view your website content.
  • Improved SEO rankingGoogle bots can crawl your site faster.

How to Defer Parsing of JavaScript in WordPress?

This section will show you a step-by-step guide to defer parsing of JavaScript. To top it off, we’ll also show you how to analyze the problem and test the changes after applying the task.

1. Analyze the site

To find out whether you should defer parsing of JavaScript in your WordPress website, analyze it using site speed testing tools like GTMetrix. As an example, here is the performance result of a website before implementing the method:

analyze defer parsing of javascript in gtmetrix

It shows that there’s an issue regarding JavaScript parsing, which means the website site needs to defer it.

2. Defer Parsing of JavaScript in WordPress

Now, let’s get into the steps of deferring parsing of JavaScript! There are two ways to do it – installing a plugin or adding a code snippet manually.

Using WordPress plugins

banner of speed booster pack

One of the easiest ways to defer JavaScript parsing is by using a plugin. Speed Booster Pack is one of the most popular plugins for this task. To begin, install and activate it, then follow these steps:

  1. Go to your WordPress Dashboard
  2. On the left sidebar, navigate to Speed Booster
  3. Click the Advanced tab, and activate Defer parsing of JS files
    defer parsing of wordpress javascript using speed booster pack plugin
  4. Once done, tap Save Changes

Defer Parsing of JavaScript via functions.php

Alternatively, you can carry out the task manually by editing one of the WordPress core files. Make sure to backup your website before proceeding. Here are the steps:

  1. From your WordPress admin area, go to Appearance -> Theme Editor
  2. Select functions.php in the Theme Files
    open theme editor to defer parsing of javascript
  3. Enter this code at the bottom of the file:
    function defer_parsing_of_js ( $url ) {
    if ( FALSE === strpos( $url, '.js' ) ) return $url;
    if ( strpos( $url, 'jquery.js' ) ) return $url;
    return "$url' defer ";
    }
    add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );
  4. Click Update File.

3. Test the Changes

After using one of the above options, re-analyze your site with GTMetrix:successful defer parsing of javascript in gtmetrix

Conclusion

In this tutorial, you’ve learned how to improve your site performance by deferring parsing of JavaScript. You can do so by using a plugin or adding a code snippet to the theme’s functions.php file.

Keep in mind that improving your website’s loading time will lead to better user experience and improved SEO ranking.

Do you have any questions regarding this topic? Share your thoughts in the comment section below!

The Author

Author

Domantas G. / @domantas

Domantas leads the content and SEO teams forward with fresh ideas and out of the box approaches. Armed with extensive SEO and marketing knowledge, he aims to spread the word of Hostinger to every corner of the world. During his free time, Domantas likes to hone his web development skills and travel to exotic places.

Related tutorials

Author

Brian Reply

April 13, 2017

Hi, thanks for the article. I tried the code both in my Wordpress wp-includes/functions.php file and my theme functions.php file and neither worked. Do you have any idea why it wouldn’t work?

    Author

    Gediminas

    Replied on April 13, 2017

    Thank you for contacting us. In some cases, the specific configuration in WordPress (.htaccess, specific plugins or other settings) may cause issues when trying to defer parsing of JS. In such cases, we would recommend deferring parsing of JavaScript via WordPress plugins. For more specific information, see Option 1 found in Step 2

Author

Candell Reply

April 13, 2017

Thanks for this, it works great with one exception, it caused Yoast SEO to break. Can we exclude Yoast from being deferred?

    Author

    Gediminas

    Replied on April 13, 2017

    It is possible to exclude scripts from being deferred when using the “Speed Booster Pack” plugin. You will be required to access the Settings section of your plug-in, and there will be a selection that says “Exclude scripts from being deferred.” There, you will need to specify the JS file that you wish to exclude. For example `jqueryfile.min.js. If you are planning on excluding more than 4 scripts, the plugin authors suggest to disable deferring parsing of JavaScript altogether, as your page speed score might be affected.

    Author

    StrangeHelper

    Replied on May 12, 2017

    you need yoast SEO plugin to work only on wp-admin section, so bypass deferring using this: if ( is_user_logged_in() ) {} else { /* content written above */ }

    Author

    GianGian

    Replied on May 18, 2017

    use this to not break Yoast SEO (it checks if user is logged in) if ( is_user_logged_in() ) {} else { function defer_parsing_of_js ( $url ) { if ( FALSE === strpos( $url, '.js' ) ) return $url; if ( strpos( $url, 'jquery.js' ) ) return $url; return "$url' defer "; } add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 ); }

Author

123project Reply

April 13, 2017

Hi I tried both codes, both of them worked but I think this method is not compatible with woocomerce because many woocomerce features not work when inserting that code!

Author

William Reply

April 24, 2017

I put in functions.php : function defer_parsing_of_js ( $url ) { if ( FALSE === strpos( $url, '.js' ) ) return $url; if ( strpos( $url, 'jquery.js' ) ) return $url; return "$url' defer "; } add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 ) It made my website crash. I had avada themes. Be careful.

    Author

    Domantas G.

    Replied on May 04, 2017

    Hello, It may be different for Avada theme, their support should provide you the code to defer parsing.

    Author

    rakesh kumar

    Replied on January 07, 2019

    Hi Basically this code loads in admin panel as well as front end. this is the reason your website crashed. It should load only on the fron-end. If you will load this only in front end then it should work.

Author

Milius Reply

April 27, 2017

Hi Sr can you make some array with this? I mean...I wanna exclude jquery. js and todo.js function defer_parsing_of_js ( $url ) { if ( FALSE === strpos( $url, '.js' ) ) return $url; if ( strpos( $url, 'jquery.js','todo.js' ) ) return $url; return "$url' defer='defer"; } add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 ); But not work

Author

Jim Miesner Reply

May 02, 2017

Hi. Thanks for this article it improved my page speed when I did this through the speed booster pack in wordpress, but then my background video didn't load. Is there a way to keep the background video active on my website and still defer parsing?

Author

Antoine en road trip Reply

May 04, 2017

It looks weird to put your code in function in wp-includes ? Are you sure it is not in function.php of the theme ?

    Author

    Domantas G.

    Replied on May 04, 2017

    Hello, Yep, you are right. Fixed!

Author

Webbing Barcelona Reply

June 21, 2017

Not working properly any of this 3 plugins :/ not even the code applied on the functions.php. The main problem is the Home slider, it doesn't appear... Any other option to solve it? Thanks!

    Author

    Domantas G.

    Replied on June 23, 2017

    Hey, Try to exclude your home slider JavaScript from deferring.

Author

Amit Kumar Reply

January 06, 2018

Hi Thanks for this article it improved my page speed when I did this through the WP deferred javaScript in wordpress.

Author

Marco Reply

January 26, 2018

Thank you. Very useful post!

Author

Anuj Godara Reply

February 26, 2018

Thanks for giving a good tips to us

Author

Bob Reply

March 25, 2018

That is amazing. I added the php code and the site went to A100% on both page speed and YSlow scores using GTmetrix. Before that it had been in the B/C range. I'd brought it up from D/C speeds by adjusting the served image sizes, but this code really made a difference. It seems to have fixed a number of other issues also, not just the defer parsing. I'm lazy so I tried the ASync plugin first, but that didn't appear to affect anything. Thank you!

    Author

    Bob

    Replied on March 25, 2018

    ok I need to retract that. The site wasn't available after I added the code. That could account for the increased speeds. Any suggestions?

Author

Mike Reply

February 26, 2019

Awesomeness!!! Keep up your excellent work! this code really works for me. Cheers, Mike

Author

Baswoko Reply

May 22, 2019

two thumb up it's add speed to my website from 57 to 70 on google speed test on mobile Thank you

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