Learn how to Defer Parsing of JavaScript to improve Page Speed score. And how you can fix ‘Eliminate render-blocking of JavaScript’ warning in Google PageSpeed Insights by deferring non-critical JavaScript(s). The newer version of Google PageSpeed Insight refers to this issue as ‘Eliminate render-blocking resources’; these render-blocking resources may include JavaScripts and CSS.
In this article, I will cover what is defer parsing of JavaScript, how to defer parsing of JavaScript properly, why you should defer parsing JavaScript, how to find render-blocking JavaScript(s) which are to be deferred, how to defer multiple JavaScripts in one go, how you can defer parsing of JavaScript in WordPress with or without plugin and how does deferred loading of JavaScript help to speed up your website?
In a nutshell, we’ll eliminate render-blocking JavaScript(s) not by actually removing (deleting) them from the website code but by defer loading them. So that they stop blocking the loading (rendering) of meaningful content (the first paint) of the website.
These terms (the above terminology) might be overwhelming for you at first, especially if you’re not a tech guy.
But, don’t worry about that!
I am going to explain everything step by step in simple words. So that you can proceed at your pace and implement the methods to fix ‘Eliminate render-blocking resources’ on your website/blog.
What is Defer Parsing JavaScript
When you test your website with pagespeed testing tools like Google PageSpeed Insights, GTmetrix, etc., you might have seen this recommendation called defer parsing of JavaScript.
So let’s find out what does it really means?
A web page is made of up several components which include HTML, CSS/Stylesheets, JavaScript, and graphical (images & icons) components etc. These components are stacked one over another in the code structure of the web page.
When a user types your website URL in the web browser’s address bar and hit enter. The browser first establishes the connection with the server on which your website is hosted.
Once the connection is established, the browser starts rendering the components of the webpage to display the web page.
The browser renders the components serially from top towards bottom of the webpage. That means what comes first rendered first and so on.
When the browser encounters a JavaScript on a web page, it downloads the JavaScript, executes it and then proceeds to render the next component. So during this time browser stop rendering rest of the web page.
Every time the browser encounters a JavaScript, it stops rendering rest of the webpage until it renders and executes the encountered JavaScript.
That’s how JavaScript blocks the critical rendering path.
To avoid this situation, Google Engineers recommend deferring non-critical JavaScript.
The question still remains the same, What is Defer Parsing of JavaScript?
Generally speaking,
Defer Parsing of JavaScript means to tell the web browser to render/download JavaScript only after the main content of the website has finished loading. This way visitors need not wait more to see the meaningful content of the website.
Technically, it is the process to defer (delay) JavaScript loading on a website.
Defer Parsing of JavaScript can be defined as the process of using
deferorasyncattribute with JavaScript to avoid render blocking of the first paint of the web page.So if you defer parsing of JavaScript that means JavaScript(s) should load, only after the meaningful content of the website has loaded.
Let’s try to put this in a perspective with Google PageSpeed Insights warning and recommendation.
When you test, a website using Google Pagespeed Insights Tool, you get some warnings and recommendations to fix those warnings/errors.
The PageSpeed Insights (PSI) text for render-blocking resources says,
Eliminate render-blocking resources.
Resources are blocking the first paint of your page. Consider delivering critical JS/CSS inline and deferring all non-critical JS/styles.
This warning triggers for two different elements i.e. JavaScript (JS) and CSS when any of them block the critical rendering path 1 during the website loading. Here in this article, we are discussing the JavaScript part.
(In the previous version of PageSpeed Insights Tool, the same warning (for the JavaScript) used to be called ‘Eliminate render-blocking of JavaScript’.)
In simple words, this warning triggers when there are some JavaScript(s) loading on your website which blocks the loading of the content that matters most to your visitors.
This means your visitors have to wait longer to see the meaningful content of your website because JavaScript(s) are blocking the rendering of content.
Clearly, Pagespeed Insights or other site speed testing tools (GTMetrix, etc.) show this warning/error, if your site loads some JavaScript(s) that block the loading of meaningful content (the first paint) of your site.
And this needs to be fixed.
Critical vs Non-critical JavaScript: Explained
As Google PageSpeed Insights (PSI) recommendation says you should deliver critical JS inline and defer all non-critical JS.
What does this mean?
Let’s break that down by terminology.
Critical JavaScripts: JavaScripts which are necessary to load during optimized critical rendering.
Non-critical JavaScripts: Those JS that can wait to load until the first meaningful content (the first paint) of the webpage has loaded.
Inline Delivery: Inline delivery refers to load a resource (in this case JS) within the HTML code instead of calling/importing that separately.
Curious? Why does JavaScript(s) block the critical rendering path in the first place?
We’ll discuss that in the next section with other reasons why you should Defer JavaScript Parsing.
Do you know? how to
- Create and Edit WordPress htaccess file – In two Simple Steps
- Leverage Browser Caching Google Analytics
Why You Should Defer Parsing of JavaScript
JavaScript Execution: is a Heavier Task
How does JS Affect the SiteSpeed?
First of all, JavaScript(s) are one of the major culprits to make your website slow.
Wondering, why is that?
Because when the web browser comes across a script, it executes the script first before continue to load HTML that includes the content users are looking for.
For a browser, executing a JavaScript is a heavier task (depending on the size of the script) and takes more time as compared to render the meaningful content (the first paint) of the webpage.
Hence JavaScript affects the critical rendering path and slows down pagespeed of your website.
Why not defer this heavier task of JS execution so that critical rendering path remain uninterrupted, right?
Pagespeed: is now a Ranking Factor
Site speed has already become a ranking signal.
About a decade ago Google announced 2 in an official blog post on Google Webmaster Central Blog that site speed has become a ranking signal.
In another blog post published on Official Webmaster Central Blog in 2018, they revealed 3 that Google started using page speed as a ranking factor in mobile search ranking.
Since Google had declared pagespeed a factor in search result rankings for desktop and mobile. Therefore, site speed optimization has become a significant aspect of technical SEO.
For the same reason, Google PageSpeed Insights Tool recommends deferred parsing of JavaScript as one of the solutions 4 to remove render-blocking JavaScript in above-the-fold content.
User Experience: decides Your Site’s Success
How does JavaScript affect user experience (UX)?
We have already discussed that JavaScript(s) slow down the pagespeed by blocking the rendering of first paint (the meaningful content). That led to more loading time and a longer wait for users to see the content; bad user experience, right.
Speed matters a lot, the truth is users do not like slow loading websites. In fact, studies show that the users leave a slow loading site early and move on.
On the contrary, you want your website audience to engage with your site and eventually turn into a customer, subscriber or ad-viewer. In order to make that happen, you need to improve your pagespeed by deferring non-critical JavaScript(s).
Reasons to Defer Loading of JavaScript: Summing it up
As I mentioned above, however, the parser (browser) gives priority to execute the script over parsing rest of the HTML, whenever it encounters the script.
But the fact is, most of the JavaScript(s) comes in use when the complete web page is loaded. For example, in some animation, effect, or some functionality, etc.
Therefore, it is a good idea to load JavaScript(s) only after the content has loaded.
This way deferred loading of JavaScript does not affect the critical render path and consequently helps to speed up your website. And hence, better user experience to your readers.
And by making your site load faster, you also improve your search ranking on desktop as well as mobile.
Do you know, a good web hosting is a must for better pagespeed?
If you are already using a good web hosting?
Awesome, let’s skip to defer parsing of JavaScript.
Not sure? whether your hosting is as good as your website deserves, don’t worry. We recommend SiteGround Hosting and Kinsta Hosting for better sitespeed.
Read our SiteGround Review and Kinsta Review.
Now, since you have an understanding of what is defer parsing of JavaScript and why should defer loading of JavaScript(s).
It is a good time to figure out which JavaScript(s) (on your website) are the culprits and need to be deferred.
If you already know which JavaScript(s) on your website are blocking the critical rendering path, you may skip the following section and jump to the implementation part. Otherwise, keep on reading…
How to Find Render-blocking JavaScript(s)
JavaScript(s) which block the rendering of meaningful content are called ‘Render Blocking JavaScript(s)’ and need to be deferred.
You can find render-blocking JavaScript(s) by analyzing your website using site speed testing tools.
There are several pagespeed testing tools available to analyze a website for site speed and loading time. I am sharing with you the most reliable and trusted tools for pagespeed testing.
Test your site using these tools and note the results of these tools so that you can compare the results before and after implementing defer parsing of JavaScript(s).
1. PageSpeed Insights by Google
Google PageSpeed Insights (PSI) is an exclusive pagespeed testing tool by Google. Test your website using Google PSI Tool to find out render-blocking JavaScript(s). PageSpeed Insights Tool results give information about warnings and their solutions/fixes.
2. GTmetrix
This one (GTmetrix) is another good free tool to test site speed. You can test your site with GTmetrix to know which JavaScripts need to be deferred.
3. Pingdom Tools
Solarwinds’ Pingdom Tools are also very popular when it comes to site speed testing tools. You can test your site using Pingdom Tools to check the number of JS requests on your site and how much they contribute to the total number of requests.
Now you know which JavaScript(s) are making your site slow and need to be deferred. So, let’s see how to fix this issue by deferring non-critical JavaScript(JS).
Test Results: Before Defer Parsing of JavaScript
I have tested a website before implementing defer parsing of JavaScript. Consider these a baseline and compare these results after defer loading of JavaScripts.
How to Defer Parsing of JavaScript [Step by step]
You need to use the following code to defer parsing JavaScript. Insert this code in HTML file just before the </body> tag. Read the instructions given below to use this script.
< script type="text/javascript">
function parseJSAtOnload() {
var element = document.createElement("script");
element.src = "script_to_be_deferred.js";
document.body.appendChild(element);
}
if (window.addEventListener)
window.addEventListener("load", parseJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", parseJSAtOnload);
else window.onload = parseJSAtOnload;
</script >
Instructions for Defer Parsing JavaScript using the script
Don’t forget to take a complete back-up before making any changes in the code. If something went wrong, you can use that back-up to go back.
- Copy the code and paste it in HTML file just before the
</body>tag (near the bottom of HTML file). - Replace script_to_be_deferred.js with the link of the JavaScript which is to be deferred. You can copy the link of JavaScript(s) (which Google PageSpeed tool suggests to defer) from Google PageSpeed Insights tool results for your website.
- Save changes. And you are done.
- Finally, test your website again to see the effect.
Code to Defer Multiple JavaScripts in One-go
If you want to defer multiple scripts in one go. You can use the same script with little modification. In the following code replace defer1.js, defer3.js, and defer3.js, etc. with the link of scripts that you want to defer.
< script type="text/javascript">
function parseJSAtOnload() {
var links = ["defer1.js", "defer2.js", "defer3.js"],
headElement = document.getElementsByTagName("head")[0],
linkElement, i;
for (i = 0; i < links.length; i++) {
linkElement = document.createElement("script");
linkElement.src = links[i];
headElement.appendChild(linkElement);
}
}
if (window.addEventListener)
window.addEventListener("load", parseJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", parseJSAtOnload);
else window.onload = parseJSAtOnload;
</script >
How to Defer Parsing of JavaScript in WordPress
You can defer parsing of JavaScript in WordPress by following methods:
- Using WordPress Plugins (with a plugin) – suitable for all the plugin lovers.
- Using the Script mentioned above (without plugin method) – suitable for geeks who don’t want to use a plugin.
- Adding a Code Snippet to function.php file – suitable for those who are used to playing with code and editing files in WordPress.
Defer Parsing of JavaScript using WordPress Plugin
There are several WordPress plugins available to defer parsing of JavaScript in WordPress, I am listing three plugins which stand out in the crowd because of their features and productivity.
- Autoptimize
- Speed Booster Pack
- W3 Total Cache
As I have explained how to properly configure these plugins for defer parsing of JavaScript in another article, therefore, I am skipping the configuration part in this article. You can check out here: How to configure Autoptimize, Speed Booster Pack, and W3 Total Cache plugin to defer parsing of JavaScript in WordPress
Defer Parsing of JavaScript without WordPress Plugin – Script Method
In WordPress, the code mentioned above can be placed in HTML file just before the </body> tag using hook content option.
Most of the popular WordPress themes come with hook content provision. If you are not using hook content option or it is not available in your theme. Then, either you can use a WordPress plugin to add the script in WordPress footer before </body> tag or you can place the script in the footer file just before the </body> tag manually.
Facing any difficulty adding the script in WordPress footer? Check out our detailed guide, to learn how to add code in WordPress header and footer easily.
Defer JavaScript Parsing in WordPress via functions.php file
Yes, you can defer parsing of JavaScript in WordPress by adding a code snippet to function.php file.
So this is another method that you can use to Defer Parsing of JavaScript in WordPress without using a plugin.
As I have mentioned above this method is suitable for people who are comfortable with code editing in WordPress.
You might be thinking, but why?
First of all, functions.php is an important theme file. That means you might end up breaking your site easily if anything went wrong with the editing of functions.php file.
Also, there are different versions of the code snippet on the web to fix defer parsing of JavaScript in WordPress via functions file. Unfortunately, not all the code snippets work fine.
So you should be careful while using a code snippet to defer loading of JavaScript.
How to Edit functions.php File Safely
I always recommend using a child theme in WordPress in order to avoid code editing mess.
Because while editing the code, even if you miss a single comma (,) semicolon (;) or any other symbol/syntax, your website will break completely or partially. And you have to make extra efforts to recover the site.
If you’re not using a child theme, learn how to use a child theme in WordPress and its benefits.
Step by step process to Defer Parsing JavaScript in WordPress via functions.php
Take a complete backup before making any changes to code.
I assume that you’re using a child theme. If you’re not, first create and activate a child theme to any trouble because of theme files editing.
- Go to your WordPress Dashboard > Appearance > Theme Editor
- Select/open functions.php file (of child theme) from theme files.
- Paste the code snippet given below at the end of functions.php file.
- You can specify JS files to exclude from defer in the array (‘jquery.js’).
- Finally, click Update File to save changes. That’s all.
The code snippet to be pasted in functions.php file.
// Defer Parsing of JavaScript in WordPress via functions.php file
// Learn more at https://technumero.com/defer-parsing-of-javascript/
function defer_parsing_js($url) {
//Add the files to exclude from defer. Add jquery.js by default
$exclude_files = array('jquery.js');
//Bypass JS defer for logged in users
if (!is_user_logged_in()) {
if (false === strpos($url, '.js')) {
return $url;
}
foreach ($exclude_files as $file) {
if (strpos($url, $file)) {
return $url;
}
}
} else {
return $url;
}
return "$url' defer='defer";
}
add_filter('clean_url', 'defer_parsing_js', 11, 1);The above code snippet is using defer attribute to defer parsing of JavaScripts. You can replace the defer attribute with async attribute to parse JavaScript asynchronously. You can read more about async attribute and other methods to fix render-blocking JavaScript.
Test Results: After Defer Parsing of JavaScript
The following are the test results after defer loading of JavaScript.
Wrapping it up
Other than defer parsing of JavaScript, you can also use async attribute or inline JavaScript to remove render-blocking JavaScript. I have covered async attribute or inline JavaScript in another blog post, read that article here. In that article, I have also mentioned a few useful WordPress plugins to defer parsing JavaScript.
Although WordPress plugins are available to defer parsing of JavaScript. The above-explained script method is considered more appropriate by several experts and webmasters. But the people who use WordPress know that using a WordPress plugin is like bliss.
I hope this guide will help you to defer parsing of JavaScript. Let me know, which technique do you use to defer parsing of JavaScript. If you are facing any problem implementing the above methods or have a question. Let me know via comment section. I will be happy to answer.
Cheers!
General FAQ
What does defer mean in JavaScript?
Defer literally means ‘hold back’ or ‘delay’. If you use ‘defer’ attribute for parsing (loading) of JavaScript, JS will execute only after the HTML parsing has finished. This helps to improve pagespeed.
How do I remove render blocking JavaScript from my website?
To remove render blocking JavaScript, you can inline critical JS and defer all non-critical JS. Everything is explained step by step in this technumero's tutorial.
How do I fix JavaScript defer parsing in WordPress?
You can do that with or without using a plugin. There are three methods to fix JavaScript defer parsing in WordPress properly: the script method, plugin method, and functions.php file method.
What's the difference between async and defer JavaScript?
The ‘defer’ attribute delays JS execution until HTML parsing has completed. However, with ‘async’ HTML parsing & script execution happens simultaneously and the script executes as soon as it’s ready.
Further readings:
- Remove Query Strings from Static Resources to Speed Up your Website
- How to Increase Page Speed using Htaccess
- Enable Keep-Alive to Speed up Your Site
- Disable Server Signature by Editing .htaccess/Apache
- Minify JavaScript and CSS in WordPress to Boost Page Speed
Awesome tutorial! Many people out there don’t know that plugins are not necessary if you want to defer JS in WordPress. We even had to publish a similar tutorial for our clients.
Hey Dominic,
Glad that you find it useful.
Thanks for stopping by.
Please help, I am confused about this html and body tags, please tell me if the body tag the one in our template?
Hey Chukwuebuka,
If you are trying to implement in WordPress, you should look for hook content feature in your theme. Under hook content, there is closing tag in the footer. Hope it helps. Thanks for stopping by.
I used this script and my website is blank i dont know what happened can you pls help me …
Hey Mithun,
To avoid such situations I always recommend to backup your files before making any changes in the code. In present situation you can access and edit the file (where you had pasted the script) via hosting cPanel > File Manager. Or using FTP client.
Hope it helps.
This is what am looking, my blog loading slow because of blocking css & js scripts. I will implement the code, hope this code will fix my blog problem. Thanks, Shashank! for sharing and expecting more posts like this. Will visit again.
Hey Charan,
Thanks for stopping by.
Looking forward to listen more from you.
Hi Shashank,
Thanks for your tutorial. My theme don’t have hook option so i tried to put the code in footer before /body but it is not working, what to do.
Do you have any code to be put in function.php with hook command for WordPress site.
Hey Vishal,
I can use one the plugins I have mentioned in this article: How to Add Code in WordPress Header and Footer. You may paste the code for defer parsing JavaScript in the footer section of the plugin.
Cheers!
Hi, thanks for the article. It worked well on my blog.
Hey Henry,
Glad that I could help.
Thanks for dropping by.
Where is this article? “How to configure Autoptimize, Speed Booster Pack, and W3 Total Cache plugin for defer parsing of JavaScript in WordPress”
The link goes to: https://technumero.com/remove-render-blocking-javascript/
Hey Bernadette,
I understand your concern. Setup of all the plugins with screen captures is in the mentioned article. Check out the lower-half of the article. Hope it helps. Thanks for stopping by!
Helllo,
Thanks for the great tutorial, but it did not help me. Do we need to delete those js lines after including them in your code?
Thanks!
Hey Fırat,
No, you need not delete JS. All you need is, to place JS path in the code and paste the code to HTML file just before the tag. You may also check this article to learn how to Add Code to WordPress Header and Footer easily.
Hope it helps. Thanks for stopping by!
Hello sir,
I am using Autoptimize, W3 Total Cache and WP Optimize plugin together.
So please tell me should we use all plugin together or not?
Sir, I have changed our theme and after changing our theme defer javascript problem enables, so how could I fixe this?
And last question is how to tick the follow button below comment box automatic Please tell me.
Hey Sumit,
Autoptimize and W3 Total Cache is used for similar purpose. You can choose one of them for your requirements because there is no point of using two different plugins for similar purpose. WP-Optimize is for WP database optimization.
As long you are NOT facing any specific issue using these plugin together, you may use them.
This article How to Add Subscribe to Comments in WordPress covers your last question.
Hi!
First thing I’ve tried to defer parsing of JavaScript was to activate the option in the cache plugin I use (Litespeed Cache), but I had some visualization problems about Rev Slider and translation plugin also(Transposh), so I excluded the corresponding JS of deferring, but not fixing the error… After that, did the same but using Fast Velocity Minify plugin instead, same results. Maybe adding the code to functions.php skipping these problematic JS could be the solution but, how can I exclude it by code?
Many thanks for the help!
Hey,
You can follow the link and instructions to setup recommended WordPress plugins to resolve your issue.
Hope that helps. Thanks for stopping by!
I just used the “Code to Defer Multiple JavaScripts in One-go” in my footer.php file and now I have a 100% score for deferring JS parsing in GTMetrix! Thanks a ton!
Hi and thanks for this article. I’ve tried to insert the code but GTmetrix continues warning me about the problem. I’ve placed it into the footer.php file of my WordPress site just before the tag, pasting the two scripts reported by the site, so I think it’s ok. Maybe I need to clear my site cache? But even if that was the case, I don’t know how to do it considering that I don’t have cache plugins.
Hey Faby,
That’s strange. The script worked fine for many WP users. If it’s not working for you for some reason, you can try a WordPress mentioned.
Thanks for stopping by!
I just implement the code on my website (html) but it doesn’t work. And I don’t see any improvement in my speed optimization, kindly suggest me that some other code if you have.
Hey Sathik,
The same worked for many readers. I’m surprised that it’s not working in your case.
Maybe take help of someone (local) with better coding skills to implement the code.
Thanks for dropping by!
I’ve used the funtions.php method for my WordPress site. It does what its meant to but changes the font of my website. Does anyone know a go-around or fix for this?
Thanks
Hi James,
You can exclude the conflicting file by adding that to the array
$exclude_files = array('jquery.js, your_file_name.js');First, you have to figure out which JS file is causing the issue. Once you identified the file, you can add that in place of your_file_name.js to exclude that from defer.
All the best!
Thanks for stopping by!