What’s the advantage of using async-js or defer-js in script loading?
Asked on Sep 28, 2025
Answer
Using "async" or "defer" attributes in script loading can significantly enhance page performance by optimizing how JavaScript files are fetched and executed. These attributes help prevent render-blocking, allowing the browser to load and display content more efficiently.
Example Concept: The "async" attribute allows scripts to be downloaded in parallel with other resources and executed as soon as they are available, which is ideal for independent scripts. The "defer" attribute ensures scripts are executed in order after the HTML is fully parsed, which is beneficial for scripts that rely on the DOM.
Additional Comment:
- Using "async" is best for scripts that do not depend on other scripts or the DOM.
- "Defer" is suitable for scripts that need to maintain execution order and interact with the DOM.
- Both attributes help reduce the time scripts block the rendering of the page.
- Implementing these attributes can improve First Contentful Paint (FCP) and Largest Contentful Paint (LCP) metrics.
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