Learn how to Defer Parsing of JavaScript to improve pagespeed 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.
Table of Contents
- What is Defer Parsing JavaScript
- Why You Should Defer Parsing of JavaScript
- How to Find Render-blocking JavaScript(s)
- Test Results: Before Defer Parsing of JavaScript
- How to Defer Parsing of JavaScript [Step by step]
- How to Defer Parsing of JavaScript in WordPress
- Test Results: After Defer Parsing of JavaScript
- Wrapping it up
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 the top towards the bottom of the webpage. That means what comes first rendered first and so on.
When the browser encounters 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 the rest of the web page.
Every time the browser encounters JavaScript, it stops rendering the 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 telling 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.
Google PageSpeed Insights 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 that 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 SiteSpeed?
First of all, JavaScript(s) is 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 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 the critical rendering path remains 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 into 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 for 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? 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… 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). 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. 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. 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). I have tested a website before implementing defer parsing of JavaScript. Consider these a baseline and compare these results after deferred loading of JavaScripts. You need to use the following code to defer parsing JavaScript. Insert this code in HTML file just before the 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. 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. You can defer parsing of JavaScript in WordPress by following methods: There are several WordPress plugins available to defer parsing of JavaScript in WordPress, I am listing the best plugins that stand out in the crowd because of their performance and reliability. Obviously, the process of installing and activating any of the following plugins remains the same. If you’re not sure about the process of installing a WordPress plugin, you can refer this beginner’s guide to learn different methods of installing a plugin in WordPress. If you want a standalone plugin to defer parsing of JavaScript, Async JavaScript should be your pick. This tiny plugin offers all necessary settings to tweak deferred loading of JS in WordPress. Steps to defer parsing of javascript in WordPress using a plugin: WP Rocket is a power-pack of features when it comes to WordPress speed optimization. You can easily defer loading of javascript using WP Rocket to speed up your site. Steps involved to enable defer loading of JS using WP Rocket plugin: DO WE USE WP-ROCKET? Hell, YES! You can read, how we got load time under 1s using WP Rocket. You can defer JavaScript loading in WordPress using W3 Total Cache plugin. Steps to defer parsing of JavaScript using W3 Total Cache WordPress plugin: LiteSpeed Cache is an amazing optimization plugin for LiteSpeed server hosting. But the general features of this plugin can be utilized on any server like LiteSpeed, Apache, NGINX, etc. Steps to defer parsing of javascript in LiteSpeed Cache plugin: Swift Performance plugin has become a well know name in the speed optimization category. Their free version is called ‘Swift Performance Lite’. The process to delay loading of JS in WordPress using Swift Performance: Speed Booster Pack also offers deferred loading of javascript out of the box. Step by step procedure to enable defer loading of js in Speed Booster Pack plugin: Autoptimize another good plugin to optimize WordPress speed. This plugin also offers the option to defer load JS in WordPress. Step by step process to defer parsing javascript using Autoptimize: You can eliminate render-blocking JavaScript resources using WP Fastest Cache plugin. But this feature is available with the premium version only. Yes, you can defer parsing of JavaScript in WordPress by adding a code snippet to function.php file. This is one of the methods 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. For any reason, if you don’t want to implement a child theme now, you can use this plugin to add code to functions.php file of your theme without editing the original file. Take a complete backup before making any changes to the 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 file editing. The code snippet to be pasted in functions.php file. 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. The script method explained above can be used in WordPress to defer loading of javascript. In WordPress, the above-mentioned code can be placed in HTML file just before the 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 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. Steps to defer load javascript in WordPress without using a plugin: The following are the test results after defer loading of JavaScript. Other than defer parsing of JavaScript, you can also use 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! Further readings: 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. 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. 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. 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.
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 Cloudways and Kinsta Hosting for better sitespeed.
Read our Kinsta Review. How to Find Render-blocking JavaScript(s)
1. PageSpeed Insights by Google
2. GTmetrix
3. Pingdom Tools
Test Results: Before Defer Parsing of JavaScript
How to Defer Parsing of JavaScript [Step by step]
</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
</body> tag (near the bottom of HTML file).Code to Defer Multiple JavaScripts in One-go
< 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
1. Defer Parsing of JavaScript using WordPress Plugin
#1.1 Async JavaScript Plugin
How to Use Async JavaScript Plugin: Settings & Usage Guide
#1.2 Defer Parsing of JavaScript Setting in WP Rocket Plugin
#1.3 Defer Parsing of JavaScript: W3 Total Cache
#1.4 Defer Loading of JavaScript in LiteSpeed Cache Plugin
#1.5 Defer Parsing of JavaScript using Swift Performance Plugin
#1.6 Defer Parsing of JavaScript in WordPress using Speed Booster Pack
#1.7 Defer Parsing of JavaScript: Autoptimize
#1.8 WP Fastest Cache to Defer Parsing of JavaScript
2. Defer JavaScript Parsing in WordPress via functions.php file
Step by step process to Defer Parsing JavaScript in WordPress via functions.php
// 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);
3. Defer Parsing of JavaScript without WordPress Plugin – Script Method
</body> tag using hook content option.</body> tag or you can place the script in the footer file just before the </body> tag manually.
</body> tag (using a plugin or built-in theme hook) .Test Results: After Defer Parsing of JavaScript
Wrapping it up
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.
General FAQ
