How to Defer Parsing of JavaScript Properly + WordPress Fix [Updated 2025 Guide]
Discover proven ways to raise your PageSpeed score by deferring the parsing of JavaScript and fixing the “Eliminate render-blocking resources” problem in Google PageSpeed Insights. The latest version of Google PageSpeed Insights has updated this warning to ‘Eliminate render-blocking resources,’ which includes both JavaScript and CSS. In this comprehensive guide, we’ll cover what deferring JavaScript means, how to do it correctly, and why it’s essential for modern site speed optimization. You’ll also learn how to identify render-blocking scripts, defer multiple JavaScripts at once, and implement solutions in WordPress—with or without plugins.
Key Takeaways: What You’ll Learn
- Deferring JavaScript = faster perceived load time: It allows critical content to render first while non‑essential scripts load later.
- Async vs. Defer: Async loads scripts during HTML parsing and executes as soon as possible; defer loads during parsing but executes only after parsing is complete.
- PageSpeed is a ranking factor: Google uses site speed for both desktop and mobile rankings.
- Multiple implementation methods: Manual script insertion, WordPress plugins (WP Rocket, Async JavaScript, etc.), or code snippets in functions.php.
- Always test before and after: Use tools like GTmetrix, Pingdom, and Google PageSpeed Insights to measure impact.
defer or async attributes to script tags so they don’t block the initial rendering of your page. For WordPress users, the easiest methods are plugins like WP Rocket, Async JavaScript, or Autoptimize. Advanced users can add custom code to functions.php. Always test with PageSpeed Insights before and after implementation.
What is Defer Parsing of JavaScript?
A web page consists of different components: HTML, CSS/Stylesheets, JavaScript, and visual elements like images and icons. These components are stacked in the code. When a user types your URL and presses enter, the browser connects to your server and begins rendering the page sequentially—from top to bottom. The browser halts rendering when it encounters a JavaScript file, fetches and runs it, and then resumes. This halt happens every time the browser meets JavaScript, blocking the critical rendering path.
To address this, Google engineers recommend deferring non‑critical JavaScript. Defer parsing of JavaScript means using the defer or async attribute to avoid blocking initial rendering. These attributes instruct the browser to parse and execute scripts either in parallel (asynchronously) or after HTML parsing is complete (defer). This way, visitors see meaningful content faster.
Difference Between Defer and Async
Both async and defer attributes allow scripts to download without blocking HTML parsing, but they execute at different times. Understanding this difference is key to proper implementation.
| Attribute | Download Behavior | Execution Timing | Best For |
|---|---|---|---|
| <script src=”script.js”> (no attribute) | Download blocks HTML parsing | Immediately after download | Not recommended for most scripts |
| <script src=”script.js” async> | Downloaded during HTML parsing | Executes as soon as downloaded, potentially before parsing finishes | Independent scripts (e.g., analytics) that don’t depend on DOM |
| <script src=”script.js” defer> | Downloaded during HTML parsing | Executes only after HTML parsing is complete | Scripts that need full DOM access (e.g., most site functionality) |
Why You Should Defer Parsing of JavaScript
⚡ JavaScript Execution is Heavy
When a browser encounters a script, it prioritizes executing it before continuing to load HTML. JavaScript execution is resource‑intensive, especially for larger scripts. Deferring non‑critical scripts allows the critical rendering path to proceed smoothly.
📈 PageSpeed is a Ranking Factor
Google officially announced site speed as a ranking signal in 2010, and in 2018 confirmed it affects mobile rankings too. Optimizing speed—including deferring JavaScript—is now essential for technical SEO.
😊 User Experience Drives Success
Slow‑loading pages frustrate users; studies show visitors abandon sites that take too long to load. By deferring JavaScript, you improve perceived performance, reduce bounce rates, and boost conversions.
How to Find Render‑blocking JavaScripts
JavaScripts that hinder the rendering of meaningful content are labeled ‘Render Blocking JavaScripts’ and should be deferred. Use these trusted tools to identify them:
Google PageSpeed Insights
Google’s official tool provides detailed warnings and lists specific scripts to defer.
GTmetrix
Analyzes your site and highlights render‑blocking resources.
Pingdom Tools
Shows JavaScript requests and their impact on load time.
Test Results: Before Deferring JavaScript
Here’s a baseline test of a website before implementing deferred JavaScript. Compare these with the after results later.
How to Defer Parsing of JavaScript [Step by Step]
Manual Script Method (Universal)
Place the following code just before the </body> tag in your HTML file. Replace script_to_be_deferred.js with the actual script URL.
<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>
Defer Multiple JavaScripts at Once
Use this modified script to defer several scripts. Replace defer1.js, defer2.js, etc., with the actual script URLs.
<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
Method 1: Using WordPress Plugins (Easiest)
Async JavaScript Plugin
Go to Plugins → Add New, search for “Async JavaScript,” install and activate. Then navigate to Settings → Async JavaScript, enable the plugin, and save. You can fine‑tune which scripts to async or defer, exclude specific scripts, and manage jQuery.
After installing WP Rocket, go to Settings → WP Rocket → File Optimization. Enable “Load JavaScript deferred” and save. This is one of the most effective caching and performance plugins.
Read our full WP Rocket review.
W3 Total Cache
Go to Performance → Minify → JS minify settings. Adjust settings as shown in the image and save. This requires careful configuration but is very powerful.
LiteSpeed Cache
Navigate to LiteSpeed Cache → Page Optimization → JS Settings. Turn on “Load JS Deferred” and save.
Swift Performance Lite
Go to Tools → Swift Performance → Settings → Optimization → Scripts. Enable “Merge Scripts,” then add scripts to defer under “Deferred Scripts.” Save changes.
Speed Booster Pack
Go to Speed Booster → Assets. Under “Optimize JavaScript,” choose “Defer” and save.
Autoptimize
Go to Settings → Autoptimize → JS, CSS & HTML. Enable “Optimize JavaScript Code,” then enable “Do not aggregate but defer” and save. Clear any cache.
Method 2: Defer JavaScript via functions.php (Code Snippet)
Important: Always use a child theme and backup your site before editing functions.php.
Add the following code to your child theme’s functions.php file. It defers all JavaScript except jQuery and scripts for logged‑in users.
// Defer Parsing of JavaScript in WordPress
// Learn more at https://technumero.com/defer-parsing-of-javascript/
function defer_parsing_js($url) {
// Add files to exclude from defer (jQuery 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);
After adding the code, click “Update File.” Test your site to confirm defer is working.
Method 3: Manual Script Insertion in WordPress (Without Plugin)
You can insert the manual script (from the universal method) into your theme’s footer.php file just before </body>, or use a “hook content” feature if your theme provides one. Alternatively, use a plugin like “Insert Headers and Footers” to safely add the code.
Pro Tip: Critical vs. Non‑Critical JavaScript
Identify which scripts are essential for above‑the‑fold rendering. Inline critical JavaScript directly in the HTML, and defer everything else. Tools like PageSpeed Insights often highlight exactly which scripts are blocking rendering.
Warning: Test Thoroughly After Changes
Deferring the wrong scripts—especially jQuery or essential plugins—can break site functionality. Always test your site thoroughly after implementing any defer method. Keep a backup and be ready to revert if needed.
Test Results: After Defer Parsing of JavaScript
Here are the same sites tested after implementing deferred JavaScript. Notice the improved scores.
Keep Reading: Master WordPress Speed Optimization
Deferring JavaScript is just one piece of the speed puzzle. Explore these related guides:
Increase WordPress Website Speed
Complete guide to faster WordPress sites.
Advanced Speed Optimization Tips
Go beyond deferring JS with advanced techniques.
Professional Speed Optimization Service
Let experts handle your site speed.
Frequently Asked Questions
A: Both allow downloading during HTML parsing. Async executes as soon as downloaded, potentially before parsing finishes. Defer executes only after HTML parsing is complete. Use defer for scripts that need full DOM access.
A: Yes, if you defer scripts that are needed for initial rendering (like jQuery). Always test after implementation and exclude essential scripts.
A: WP Rocket is the most user‑friendly premium option. Among free plugins, Async JavaScript and Autoptimize are excellent.
A: PageSpeed Insights also flags render‑blocking CSS. You can inline critical CSS and defer non‑critical stylesheets.
A: Use your browser’s DevTools (Network tab) to see if scripts load with the “defer” attribute. Also, re‑test with PageSpeed Insights or GTmetrix.
Wrapping It Up
Deferring parsing of JavaScript is one of the most impactful speed optimizations you can perform. Whether you choose a plugin for convenience or a manual code method for fine control, the result is faster perceived load times, better user experience, and improved SEO rankings. Remember to always test before and after, and exclude any scripts critical to your site’s initial functionality.
If you have questions or run into issues implementing these methods, leave a comment below—we’re happy to help!
Ready to Supercharge Your Site Speed?
Start with a free performance audit or explore our professional optimization services.
Learn how to improve your page speed score by deferring the parsing of JavaScript
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This is a really helpful and comprehensive guide on deferring JavaScript! I especially appreciate the clear explanations of defer vs. async and the step-by-step instructions for WordPress. It’s great to see different methods, from plugins to manual code implementation.
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this is exactly the kind of technical guide i’ve been looking for! i’ve been struggling with page speed issues on my wordpress site for months, and i knew javascript was part of the problem but didn’t understand the difference between deferring and async loading until now. the explanation about render-blocking resources finally clicked for me.
i especially appreciated the step-by-step breakdown – as someone who’s not a developer, i found the wordpress-specific section really helpful. i was able to implement the changes using one of the recommended plugins, and my gtmetrix score actually improved by about 15 points!
one question though – after deferring javascript, should i be concerned about any functionality breaking on my site? i’ve heard stories about deferred scripts causing issues with analytics tracking or interactive elements. would love to hear your thoughts on what to watch out for after making these changes.
thanks for keeping this updated for 2025 too – it’s clear you put a lot of work into making this comprehensive. bookmarked for future reference!
I’m so glad this guide helped clarify things for you! 🎉 It’s great to hear your GTmetrix score improved after implementing the plugin changes.
You’re right to be cautious—deferring JavaScript can sometimes affect functionality, especially for analytics scripts, sliders, pop-ups, or other interactive elements that rely on JS loading immediately. A few things to watch for:
Test key interactive features on your site after enabling defer/async.
Check that tracking scripts (like Google Analytics or Tag Manager) are still firing correctly.
If certain elements break, most plugins let you exclude specific scripts from being deferred.
Taking it step by step and monitoring your site is usually enough to avoid major issues. Sounds like you’re already on top of it, so you should be fine!