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Async vs. Defer: A Simple Explanation of Script Loading

Linda Hamilton
Release： 2024-11-08 08:29:02
Original
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When it comes to loading JavaScript in a website, understanding how different loading methods can impact the performance and behavior of your website is important. JavaScript can be loaded in various ways, primarily using the default loading method, async, and defer. Each of these methods has its own characteristics and use cases. In this post, we’ll explore these three methods to help you make informed decisions for your projects.

Async vs. Defer: A Simple Explanation of Script Loading

Default Loading

By default, JavaScript files are loaded synchronously when included in an HTML document. This means that the browser will pause parsing the HTML document to download and execute the JavaScript file before continuing.

Here is how you typically include a script using the default method:

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

The async attribute allows the browser to download the JavaScript file asynchronously while it continues to parse the HTML document. Once the script is downloaded, it is executed immediately, potentially before the HTML parsing is complete.

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

The defer attribute also downloads the JavaScript file asynchronously, but the key difference is that the script is executed only after the HTML document has been fully parsed.

<script src="script.js" defer></script>
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Comparing the Three Methods

Async vs. Defer: A Simple Explanation of Script Loading

Quick Tips

  1. Use async for Non-Critical Scripts: For scripts like analytics, advertisements, and other third-party integrations that do not depend on the DOM.
  2. Use defer for DOM-Dependent Scripts: For scripts that manipulate the DOM or need to be executed in a specific order.
  3. Minimize Default Script Loading: Avoid the default synchronous loading for large scripts or scripts that can be loaded asynchronously.
  4. Load Scripts at the Bottom of the Body: If you must use the default loading behavior, place your
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