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Removing Blocking JavaScript for Faster Rendering


Fast rendering is crucial for providing an enhanced user experience on websites. When a page loads quickly, users are more likely to stay engaged and explore further. However, one common factor that can significantly slow down rendering is blocking JavaScript.

Blocking JavaScript refers to JavaScript code that prevents the browser from rendering the page until it has finished executing. This delay can have a negative impact on the overall page load time, resulting in a slower user experience.

The objective of this blog post is to explore various methods for removing or optimizing blocking JavaScript to achieve faster rendering. By implementing these techniques, website owners can improve the performance of their sites and deliver a seamless browsing experience to their users.

Understanding Blocking JavaScript

Blocking JavaScript refers to JavaScript code that prevents the rendering of a web page from proceeding until the code is fully executed. This can significantly delay the rendering process and impact page load time.

When a browser encounters blocking JavaScript, it stops parsing the HTML and executes the JavaScript code before continuing with the rendering. This can cause a delay because JavaScript execution is single-threaded, meaning that it can only process one task at a time.

Common scenarios where blocking JavaScript negatively affects rendering include:

  • Inline JavaScript: When JavaScript code is embedded directly within the HTML file, it is executed synchronously, blocking the rendering process until the code is fully executed.
  • External JavaScript: When a web page references an external JavaScript file using a <script> tag, the browser has to fetch and execute the file before continuing with the rendering.
  • JavaScript in the <head> section: Placing JavaScript code in the <head> section of an HTML file can delay rendering since the browser needs to execute the JavaScript before rendering the rest of the page.
  • Large JavaScript files: If a JavaScript file is large and contains complex logic, it can take longer to execute, causing a delay in rendering.

In all these scenarios, the presence of blocking JavaScript hampers the rendering of the web page, resulting in slower load times and a negative impact on the user experience.

Impact of Blocking JavaScript on Performance

When it comes to website performance, the impact of blocking JavaScript cannot be underestimated. Slower rendering due to blocking JavaScript can have a direct correlation with higher bounce rates. Users expect websites to load quickly, and if they experience delays, they are more likely to abandon the page.

Not only does blocking JavaScript affect user experience, but it also has a significant impact on search engine rankings. Page load time is one of the key factors considered by search engines when determining the ranking of a website. If your website takes too long to load due to blocking JavaScript, it can negatively affect your search engine optimization (SEO) efforts and make it harder for users to find your site.

Studies have shown that there is a clear connection between faster page load times and improved user engagement. For example, a study by Google found that as page load time increases from 1 second to 5 seconds, the probability of bounce rate increases by 90%. Another study by Akamai revealed that even a mere 100-millisecond delay in page load time can result in a 7% reduction in conversions.

These statistics and studies highlight the importance of removing or optimizing blocking JavaScript for performance optimization. By minimizing the impact of blocking JavaScript on rendering speed, you can provide a better user experience, reduce bounce rates, and improve your search engine rankings.

Techniques to Remove or Optimize Blocking JavaScript

When it comes to improving rendering speed, removing or optimizing blocking JavaScript is crucial. Here are some effective techniques to achieve faster rendering:

1. Deferred Loading

Deferred loading allows JavaScript files to be loaded asynchronously, which reduces their impact on rendering speed. This technique is particularly useful for non-critical scripts that can be loaded after the initial rendering.

There are multiple ways to implement deferred loading. One approach is to use the async or defer attributes in the HTML script tag. For example:

<script src="script.js" async></script>

Alternatively, you can use JavaScript libraries like loadJS or RequireJS to dynamically load scripts after the initial rendering. Here's an example using loadJS:


By deferring the loading of non-critical JavaScript, the rendering process can proceed without unnecessary delays.

2. Lazy Loading

Lazy loading is a technique that selectively loads JavaScript only when it is needed. This approach is especially beneficial for heavy JavaScript files that are not required for the initial rendering.

To implement lazy loading, you can use libraries like LazyLoad or Intersection Observer API. These libraries allow you to load JavaScript files when specific conditions are met, such as when an element becomes visible in the viewport.

const lazyLoadInstance = new LazyLoad({
    elements_selector: ".lazy"

By lazy loading JavaScript, you can significantly reduce the initial rendering time, as non-essential scripts are only loaded when necessary.

3. Asynchronous JavaScript Execution

Asynchronous JavaScript execution allows scripts to run independently, without blocking other processes. By modifying existing JavaScript code to execute asynchronously, you can prevent it from interfering with rendering.

To execute JavaScript asynchronously, you can use techniques like callbacks, promises, or async/await. For instance, you can use the setTimeout function to delay the execution of a script:

setTimeout(function() {
    // Code to be executed asynchronously
}, 0);

By executing JavaScript asynchronously, you ensure that rendering is not hindered by blocking scripts.

4. Prioritizing and Minifying Critical JavaScript

To optimize rendering speed, it is essential to prioritize critical JavaScript files and minify them. Critical JavaScript refers to code that is necessary for the initial rendering and functionality of the page.

Prioritizing critical JavaScript involves loading it first or inline in the HTML, ensuring that it is processed before other non-critical scripts. By doing so, you minimize the impact of blocking scripts on rendering.

Minifying JavaScript involves removing unnecessary characters, comments, and whitespace, reducing the file size. This results in faster loading and rendering times.

To minify JavaScript, you can use various tools like UglifyJS, Terser, or online minification services.

By prioritizing and minifying critical JavaScript, you can optimize rendering speed and improve the overall performance of your web page.

1. Deferred Loading

Deferred loading is a technique that delays the loading and execution of JavaScript until after the initial rendering of a webpage. This allows the browser to prioritize rendering the visible content to the user before loading and executing JavaScript.

By deferring the loading of JavaScript, the browser can avoid the blocking behavior that occurs when JavaScript is executed synchronously. This results in faster rendering and improved user experience.

There are multiple ways to implement deferred loading. One approach is to use the defer attribute in the script tag of the HTML document. This attribute tells the browser to defer the execution of the JavaScript until the document has finished parsing.

<script src="script.js" defer></script>

Another approach is to use JavaScript libraries, such as RequireJS or LazyLoad, that provide more advanced deferred loading capabilities. These libraries allow you to specify dependencies and load JavaScript modules asynchronously when needed.

Here is an example using RequireJS:

<script src="require.js" data-main="main.js"></script>

In the main.js file, you can define your modules and their dependencies:

require(['module1', 'module2'], function(module1, module2) {
  // Code that depends on module1 and module2

This way, the browser can load and execute the JavaScript modules asynchronously, improving rendering speed.

Deferred loading can significantly speed up rendering, especially for large JavaScript files or when multiple scripts are used on a webpage. By deferring JavaScript execution, the browser can focus on rendering the content visible to the user, resulting in a faster and more responsive experience.

2. Lazy Loading

Lazy loading is a technique that allows you to selectively load JavaScript when it is needed, instead of loading everything upfront. This can significantly improve rendering speed and enhance the overall user experience.

The main advantage of lazy loading is that it reduces the initial page load time by deferring the loading of non-critical JavaScript until it is required. By loading JavaScript only when necessary, you can prioritize the rendering of essential content, ensuring a faster and smoother initial page display.

Implementing lazy loading can be done using libraries or custom coding approaches. There are several JavaScript libraries available that simplify the process of lazy loading, such as LazyLoad, Echo.js, and LazySizes. These libraries provide easy-to-use functions or attributes that you can add to your HTML elements to trigger the loading of JavaScript files only when they become visible on the screen.

Alternatively, you can implement lazy loading using custom coding approaches. This involves adding event listeners to specific actions, such as scrolling or clicking, to load JavaScript files dynamically. By dynamically loading JavaScript based on user interactions, you can further optimize rendering speed and reduce the overall page load time.

Overall, lazy loading is a powerful technique for optimizing rendering speed by selectively loading JavaScript when needed. Whether you choose to use libraries or custom coding approaches, implementing lazy loading can greatly improve the performance of your web pages.

3. Asynchronous JavaScript Execution

When JavaScript code is executed synchronously, it blocks the rendering process, resulting in slower page load times. Asynchronous JavaScript execution allows the browser to continue rendering the page while the JavaScript code is being loaded and executed in the background.

To modify existing JavaScript code to execute asynchronously, you can use the async attribute in script tags. This attribute tells the browser to fetch and execute the script asynchronously, without blocking the rendering process. Here's an example:

<script async src="script.js"></script>

In this example, the script.js file will be fetched asynchronously while the page continues to render. Once the file is fetched, it will be executed without blocking the rendering process.

Another approach is to use the defer attribute, which also allows asynchronous execution but guarantees that the script will be executed in the order they appear in the HTML document. Here's an example:

<script defer src="script.js"></script>

In this case, the script will be fetched asynchronously, but its execution will be deferred until the HTML parsing is complete. This can be useful when the order of script execution is important.

For popular JavaScript functionalities, there are specific asynchronous execution patterns that can be used. For example, when making AJAX requests, you can use the fetch API or asynchronous functions with async/await syntax. Here's an example using the fetch API:

  .then(response => response.json())
  .then(data => {
    // Handle the data asynchronously
  .catch(error => {
    // Handle errors asynchronously

By using asynchronous execution patterns, you allow the browser to continue rendering the page while waiting for the response from the server. This can significantly improve the perceived performance of your web application.

In conclusion, asynchronous JavaScript execution allows the browser to render the page without being blocked by JavaScript code. Modifying existing JavaScript code to execute asynchronously by using the async or defer attributes, as well as using specific asynchronous execution patterns for popular JavaScript functionalities, can greatly improve rendering times and enhance the overall user experience.

4. Prioritizing and Minifying Critical JavaScript

When it comes to optimizing JavaScript for faster rendering, prioritizing critical code and minifying the JavaScript files are essential steps. By prioritizing critical JavaScript, we ensure that the necessary code is loaded and executed first, allowing the browser to render the page faster. Minifying JavaScript reduces its file size by removing unnecessary characters, such as whitespace and comments, resulting in faster loading and rendering.

Prioritizing critical JavaScript is important because it allows us to load and execute the most important code first. This ensures that the vital functionality of the page is available to the user as soon as possible. To identify critical JavaScript code, we can analyze the page and determine which functionalities are necessary for the initial rendering. For example, if a navigation menu or a search bar is essential for the user to interact with the page, the code responsible for these components should be prioritized.

To prioritize critical JavaScript, techniques such as code splitting or asynchronous loading can be employed. Code splitting involves dividing the JavaScript code into smaller chunks and dynamically loading only the necessary chunks for the initial rendering. Asynchronous loading allows the browser to load JavaScript in the background while rendering the rest of the page, ensuring that critical code is executed as soon as it becomes available.

Minifying JavaScript provides several benefits for faster loading and rendering. By removing unnecessary characters and reducing the file size, minified JavaScript can be downloaded and parsed by the browser more quickly. This results in faster loading times, especially for users with slower internet connections. Additionally, minified JavaScript reduces the amount of data that needs to be transferred, leading to improved performance and a better user experience.

To minify JavaScript, various tools and libraries are available that automatically remove whitespace, comments, and other unnecessary characters. These tools can be integrated into the build process of a website or used as standalone solutions. Additionally, minification can be combined with other optimization techniques, such as compression and caching, to further improve the loading and rendering speed of JavaScript files.

By prioritizing critical JavaScript and minifying the code, we can significantly improve the rendering speed of web pages. These techniques ensure that the necessary code is loaded and executed promptly, allowing the browser to render the page faster. Minifying JavaScript reduces the file size and improves loading times, resulting in a better user experience.

Performance Testing and Optimization Tips

Performance testing is crucial in measuring the impact of JavaScript optimization techniques and ensuring that rendering improvements are achieved. By conducting performance tests, developers can identify areas of improvement and assess the effectiveness of the optimization techniques implemented.

There are several tools and methods available for performance testing and assessing rendering improvements. One popular tool is Google Lighthouse, which provides insights into various performance metrics, including page load time, time to interactive, and render-blocking resources. Lighthouse also offers suggestions for optimization based on best practices.

Another useful tool is WebPageTest, which allows developers to test their web pages from different locations and devices. It provides detailed performance data, including waterfall charts, which help identify any rendering bottlenecks or blocking JavaScript.

In addition to using performance testing tools, developers can optimize their JavaScript code further by following these tips:

  1. Minify JavaScript: Minifying JavaScript reduces file size by removing unnecessary characters and spaces. This optimizes the loading time and speeds up rendering.

  2. Use a Content Delivery Network (CDN): Utilizing a CDN helps distribute JavaScript files across multiple servers geographically closer to the users. This reduces latency and improves rendering speed.

  3. Combine and bundle JavaScript files: Combining multiple JavaScript files into a single file reduces the number of requests and minimizes the overhead of loading separate files.

  4. Use browser caching: Leveraging browser caching allows JavaScript files to be stored locally, reducing the need for repeated downloads. This improves rendering speed for returning visitors.

  5. Optimize images: Optimizing images by compressing them and using the appropriate formats (e.g., JPEG, PNG, or SVG) reduces file size and improves rendering speed.

By implementing these tips and conducting performance testing, developers can optimize their JavaScript code and achieve faster rendering, resulting in an improved user experience.


In this article, we have explored the significance of removing or optimizing blocking JavaScript for faster rendering and improved user experience. We discussed the impact of blocking JavaScript on page load time and its correlation with slower rendering and higher bounce rates. We also highlighted the importance of page load time in search engine rankings.

To achieve faster rendering, we discussed several techniques that can be used to remove or optimize blocking JavaScript. Deferred loading allows JavaScript to be loaded after the initial rendering, improving the overall performance. Lazy loading selectively loads JavaScript when needed, reducing the initial page load time. Asynchronous JavaScript execution modifies existing code to allow for non-blocking execution, improving rendering speed. Prioritizing and minifying critical JavaScript also contributes to faster loading and rendering.

We emphasized the importance of performance testing to measure the impact of JavaScript optimization techniques. By using tools and methods for performance testing, developers can assess rendering improvements and further optimize their JavaScript code.

In conclusion, removing or optimizing blocking JavaScript is crucial for achieving faster rendering and enhancing the user experience. We encourage readers to implement the techniques discussed in this article and continuously optimize their JavaScript code to improve performance. By doing so, they can ensure that their websites load quickly and provide a seamless browsing experience for their users.


Here are some relevant sources, studies, and articles that can provide further information on the topic of removing blocking JavaScript for faster rendering:

  1. "Optimizing JavaScript Execution" - Google Developers - link

  2. "Improving Web Performance with JavaScript Loading Strategies" - Smashing Magazine - link

  3. "Optimizing JavaScript for Performance" - Mozilla Developer Network - link

  4. "The Impact of JavaScript on SEO" - Search Engine Journal - link

  5. "JavaScript Loading Strategies and Performance" - CSS-Tricks - link

Please note that these references are a starting point and there are many more resources available online that can provide additional insights and techniques for removing blocking JavaScript and optimizing rendering speed.