3 min read performance

The High-Stakes Game of Page Loading with Async and Defer

Explore the impact of 'async' and 'defer' attributes in script loading on web page performance, with insights into their benefits, use cases, and influence on SEO.

Optimizing Script Loading: Mastering Async and Defer in Web Development

Imagine a web page as a bustling city. Every script is like a construction project in this city. Traditional script loading is akin to closing all roads until a project finishes. This is where async and defer come in, changing the traffic rules for these projects.

The Traditional Way: A Traffic Jam

Without async or defer, scripts are like roadblocks. Here’s a typical script tag:

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

In this scenario, the browser hits the script and stops building (rendering) the page until the script is fully loaded and executed. This is a major delay in page loading, especially if the script is large or the server is slow.

Async: The Speedy Delivery

Using async is like using a helicopter to deliver construction materials. The city (page) construction doesn’t stop; the delivery (script loading) happens in parallel. Once the materials (script) arrive, they’re immediately used (executed).

Here’s how you’d use async:

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

The browser continues to parse the HTML while the script is being fetched. Once it’s available, it pauses parsing to execute it. It’s great for scripts like analytics, where order doesn’t matter.

Defer: The Night Shift

defer is like scheduling construction work at night. The day’s hustle and bustle (page rendering) goes on, and the construction (script execution) happens after all daily activities (parsing) end.

Here’s the defer in action:

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

Deferred scripts are fetched in parallel with HTML parsing but executed only after the document is completely parsed. This is ideal for scripts that need the entire DOM and don’t depend on other scripts.

Real-World Example

Consider a webpage with three scripts:

  1. analytics.js (Analytics script, independent)
  2. jquery.js (Library, needed for script 3)
  3. app.js (Your application script, depends on jQuery)

Here’s how you might include them:

<script async src="analytics.js"></script>
<script defer src="jquery.js"></script>
<script defer src="app.js"></script>

analytics.js is loaded with async since it’s independent. jquery.js and app.js are deferred, ensuring jQuery loads before app.js.

Impact on SEO

While Google doesn’t directly factor async and defer into SEO, they contribute to faster page loads, improving user experience and thus, indirectly benefiting SEO rankings.

Advanced Considerations

  1. Combining Async with Preloading: For critical scripts, use <link rel="preload" as="script" href="script.js"> with async to prioritize fetching.
  2. Dynamically Inserted Scripts: Scripts added via JavaScript are async by default. Keep this in mind for dynamically loaded content.


Using async and defer strategically can significantly improve page load times, enhancing user experience and potentially boosting SEO. However, it’s essential to understand the dependencies and execution order of your scripts to make the right choice.

Fun Fact: defer has been around since HTML 4.01, a testament to the forward-thinking of early web standards, though it took browsers a while to catch up. It’s like finding out your old flip phone had a feature you never knew about! 📱✨

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