Async vs Defer Loading

Introduction

When integrating third-party scripts into your web applications, loading strategies are crucial for performance. The two primary methods for loading external scripts are async and defer. Understanding these methods helps ensure better page load times and user experiences.

Async Loading

Using the async attribute allows a script to be downloaded asynchronously while the HTML document continues to be parsed. This means that the script will execute as soon as it is downloaded, which can lead to the script executing before the document is fully parsed.

<script src="https://example.com/script.js" async></script>

Defer Loading

The defer attribute indicates that the script should be executed after the document has been fully parsed. This ensures that the scripts will execute in the order they are included in the document.

<script src="https://example.com/script.js" defer></script>

Comparison

Best Practices

• Use async for scripts that are independent and do not rely on DOM elements.
• Use defer for scripts that must access DOM elements or depend on other scripts.
• Load scripts at the end of the body to improve initial page load time.
• Consider using a content delivery network (CDN) for faster loading of third-party libraries.

FAQ