When it comes to SEO and the loading of external JavaScript scripts, neither defer nor async directly impacts SEO rankings. However, both attributes affect how quickly your page loads, which can indirectly influence SEO, since page speed is one of the ranking factors that Google considers.
Here’s a breakdown of how each option works and which might be better depending on your goals:
1. async Attribute
- How it works: The async attribute loads the script asynchronously, meaning the HTML document continues parsing while the script is being fetched and executed. Once the script is fully loaded, it interrupts the document parsing and executes.
- Pros:
- Useful for independent scripts that don't rely on other scripts or DOM elements.
- Can speed up page loading time if the scripts are large or not critical.
- Cons:
- Scripts loaded with async may execute out of order, which can lead to issues if one script depends on another.
- SEO Impact:
- Since async scripts do not block page rendering, it can help improve perceived load time and user experience, which indirectly boosts SEO.
2. defer Attribute
- How it works: The defer attribute ensures that the script is downloaded while the HTML document is parsing but only executed once the HTML parsing is complete (i.e., after the entire document has been parsed).
- Pros:
- Ideal for scripts that rely on the DOM being fully loaded before execution (e.g., when modifying elements on the page).
- Ensures the scripts execute in order.
- Does not block the page from rendering while the script is being fetched.
- Based on caniuse.com, defer has more coverage over browsers
- SEO Impact:
- Like async, defer improves page load performance by allowing the page to render while scripts are downloaded, thus helping with core web vitals, like Largest Contentful Paint (LCP) and First Contentful Paint (FCP), which are important ranking signals.
Async vs Defer diagram on how HTML content is parsed
| |
Async |
Defer |
| Script is download at the background at low priority with no render blocking |
✅ |
✅ |
| Interrupt DOM Rendering to execute |
✅ |
❌ |
| Executes immediately once it is loaded |
✅ |
❌ |
| Executes just before the DOMContentLoaded event |
❌ |
✅ |
| Executes in sequence |
❌ |
✅ |
<body>
<!-- ... content -->
<!-- file will be downloaded at low priority -->
<!-- and executed immediately once it is loaded -->
<script async src="async-script-file.js"></script>
<!-- file will be downloaded at low priority -->
<!-- and executed before the DOMContentLoaded event -->
<script defer src="defer-script-file.js"></script>
</body>
Examples for Async and Defer attributes on Script tag
Which to Use for Better SEO?
For SEO purposes, using the defer attribute is generally better if your scripts depend on the DOM being ready or if they must be executed in order. This ensures that the page content is rendered and available to the user (and Googlebot) before any script execution begins.
Use defer when:
- The script needs to interact with the DOM.
- The script should execute after the entire page is parsed.
- Script order matters (e.g., libraries or other dependent scripts).
Use async when:
- The script is independent and doesn’t affect the DOM.
- The script can run at any time and doesn't need to wait for the page content to be fully parsed.
In both cases, the main SEO benefit comes from faster page rendering, which improves user experience. Faster loading times are a direct ranking factor, as Google values sites that provide a smooth and quick experience for users.
Conclusion:
- defer is typically better if you want to ensure that the scripts don’t interfere with the page rendering process and you need the DOM ready before execution.
- async can be helpful for independent scripts that don’t affect the rest of the page, but it can lead to unpredictable behavior if script order matters.
Both attributes can be used strategically to optimize your page load time, which contributes positively to your site's SEO performance.
