Defer Loaded JavaScript Files with Inline JavaScript

Introduction

In modern web development, enhancing page load performance is crucial for both user experience and SEO. One effective technique is deferring JavaScript files loaded in the header of your HTML document. By deferring these scripts, you ensure they execute only after the HTML document has been fully parsed, resulting in faster initial page load times. This approach can particularly improve scores on tools like Google PageSpeed Insights, GTmetrix, and Pingdom Tools.

I’ll show you how to use inline JavaScript to defer all JavaScript files loaded in the header. I’ll also provide an example where you can selectively defer certain scripts. These methods will help you optimize your web pages, leading to better performance metrics and happier users.

Defer All Loaded JavaScript Files

Let’s start by deferring all JavaScript files already loaded in the header of your HTML document. By adding a small inline JavaScript snippet, you can dynamically set the defer attribute for all script tags found in the header.

Here’s an example HTML structure with the inline JavaScript:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Defer All JS Example</title>
    <script src="script1.js"></script>
    <script src="script2.js"></script>
    <script src="script3.js"></script>
</head>
<body>
    <h1>Hello World</h1>

    <script>
        document.addEventListener("DOMContentLoaded", function() {
            const scripts = document.querySelectorAll('head script[src]');
            scripts.forEach(script => {
                script.setAttribute('defer', 'defer');
            });
        });
    </script>
</body>
</html>

Explanation:

Event Listener: The script adds an event listener for the DOMContentLoaded event, ensuring the code runs only after the entire HTML document has been loaded and parsed.
Script Selection: Using document.querySelectorAll('head script[src]'), it selects all <script> tags within the <head> that have a src attribute.
Setting Defer Attribute: It loops through each selected script and sets the defer attribute, causing the script to execute after the document is fully parsed.

Defer Selected JavaScript Files

Sometimes, you may only want to defer specific JavaScript files rather than all of them. This can be useful if you have certain scripts that need to load earlier for functionality reasons. Here’s how you can defer only selected scripts:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Defer Selected JS Example</title>
    <script src="script1.js"></script>
    <script src="script2.js"></script>
    <script src="script3.js"></script>
</head>
<body>
    <h1>Hello World</h1>

    <script>
        document.addEventListener("DOMContentLoaded", function() {
            const scriptsToDefer = ['script1.js', 'script3.js'];
            const scripts = document.querySelectorAll('head script[src]');
            scripts.forEach(script => {
                if (scriptsToDefer.includes(script.src.split('/').pop())) {
                    script.setAttribute('defer', 'defer');
                }
            });
        });
    </script>
</body>
</html>

Explanation:

Event Listener: As before, the script runs after the DOM is fully loaded.
Define Scripts to Defer: An array scriptsToDefer contains the filenames of the scripts you want to defer.
Conditional Defer: The script loops through each <script> tag, and if the script’s src attribute matches any in the scriptsToDefer array, it sets the defer attribute.

Practical Application

Deferring JavaScript can significantly improve your webpage’s load performance. By ensuring that scripts execute after the document is fully parsed, you reduce the initial load time, making your site feel faster for users. This leads to better performance scores in tools such as Google PageSpeed Insights, GTmetrix, and Pingdom Tools.

To verify the impact of deferring your JavaScript files, follow these steps:

Measure Baseline Performance:

Before making any changes, run your webpage through performance tools like Google PageSpeed Insights, GTmetrix, or Pingdom Tools to get a baseline performance score.

Implement the Defer Script:

Use one of the provided code snippets to defer your JavaScript files.

Re-measure Performance:

After implementing the defer script, re-run your webpage through the same performance tools to compare the results.

Analyze Results:

Look for improvements in metrics such as page load time, time to interactive, and overall performance scores.

Conclusion

Deferring JavaScript files loaded in the header of your HTML document can lead to significant performance improvements. Whether you choose to defer all scripts or selectively defer specific ones, these techniques will help you optimize your webpages effectively. By following the practical steps and verifying results using tools like Google PageSpeed Insights, GTmetrix, and Pingdom Tools, you ensure your optimizations lead to tangible benefits. Try out these methods, measure the impact, and enjoy a faster, more responsive website. If you have any questions or need further assistance, feel free to leave a comment below. Happy coding!

Questions and Answers

Q: Can I defer inline scripts using this method?
A: No, this method only applies to external scripts loaded with the src attribute. Inline scripts cannot be deferred using the defer attribute. If you need to defer inline scripts, consider wrapping them in a function and calling that function after the page has loaded.

Q: What happens if I try to defer scripts that are already deferred?
A: Adding the defer attribute to scripts that are already deferred has no additional effect and is harmless. The scripts will continue to execute in the same manner as before.

Q: Will this affect scripts loaded in the body?
A: No, the script provided in the examples only targets scripts loaded in the header. Scripts loaded in the body will not be affected by this code.

Q: Can I use this approach to defer scripts conditionally based on other criteria?
A: Yes, you can modify the condition in the if statement to defer scripts based on other attributes or criteria. For example, you could defer scripts based on their file size, a custom attribute, or even the time of day.

Q: Is this method SEO-friendly?
A: Yes, deferring scripts can improve page load speed, which is beneficial for SEO. Faster page loads contribute to a better user experience and can positively impact your site’s search engine ranking. Additionally, tools like Google PageSpeed Insights consider deferred scripts as a performance improvement.

Related Subjects

Optimizing Page Cache for Magento 2 with .htaccess

JavaScript Performance Optimization:
Learn and implement various techniques to optimize JavaScript loading and execution, significantly enhancing web performance. Check out resources like Google Developers and Mozilla Developer Network.
Understanding the defer Attribute:
Dive deeper into the defer attribute, its benefits, and how it compares to other methods like async for loading scripts. Find detailed explanations on MDN Web Docs.
Page Load Performance:
Explore comprehensive strategies to improve page load performance, including lazy loading, caching, and minimizing render-blocking resources. Access helpful guides on W3C Web Performance.
DOM Manipulation with JavaScript:
Master the basics and advanced techniques of DOM manipulation using JavaScript to create dynamic and responsive web pages. Learn from detailed tutorials on JavaScript Info and W3Schools.

These related subjects will provide you with a broader understanding and additional tools to enhance your web development skills.