Async vs. Defer: Which HTML5 Script Loading Attribute Optimizes Your Website Best?

Async vs. Defer: Optimizing Script Loading in HTML5

Understanding Async and Defer

The async and defer attributes provide mechanisms to control the loading and execution order of external scripts in HTML5 browsers.

Async Attribute

• Scripts with async execute immediately when loaded.
• Execution order is not guaranteed. For instance, a script added later can execute before one added earlier.
• Useful for non-blocking scripts that do not rely on other script executions.

Defer Attribute

• Scripts with defer also load asynchronously but only execute after the initial HTML parsing is complete.
• Ensures the execution order matches the order of appearance in the page.
• Suitable for scripts that are not critical for page rendering.

Impact on Site Load Speed

Bottom of the Page:

• Adding async to scripts at the bottom of the page may slightly improve load speed by allowing scripts to load concurrently.

Top of the Page:

• Moving scripts with async or defer to the can speed up rendering by freeing up the main thread for other tasks.
• However, HTML4 browsers may experience delays.

defer vs. Bottom of the Page Placement

• Using defer inside can achieve similar results to placing scripts before .
• defer guarantees execution order, while scripts placed at the end of the document do not.

Using async with Multiple Scripts

• Scripts with async can download concurrently.
• But execution order is not guaranteed, leading to potential issues if scripts depend on one another.
• Ensure scripts do not have dependencies or handle potential race conditions.

Defer vs. Async vs. Status Quo

• Defer preserves execution order, making it suitable for most cases.
• Async provides slightly faster loading, but comes with the potential for unordered execution.
• Until HTML5 is widely adopted, consider delaying implementation to avoid compatibility issues in HTML4 browsers.

The above is the detailed content of Async vs. Defer: Which HTML5 Script Loading Attribute Optimizes Your Website Best?. For more information, please follow other related articles on the PHP Chinese website!