Async Defer Attributes
Async and defer are HTML attributes used with <script> tags to control how external JavaScript files are loaded and executed in web browsers. Async allows scripts to download asynchronously without blocking HTML parsing and execute immediately upon download completion, while defer downloads asynchronously but delays execution until after HTML parsing is complete. These attributes optimize page loading performance by managing script execution timing.
Developers should use async for scripts that are independent and don't rely on DOM content, such as analytics or ads, to prevent render-blocking. Defer is ideal for scripts that depend on the full DOM being ready, like jQuery plugins or interactive components, ensuring they run in order after parsing. Both improve page load speed and user experience by reducing blocking behavior.