The difference between sync, defer, and async in JS

1. <script src="script.js"></script>

   No defer or async , the browser defaults to sync sync, loading and executing the specified script immediately, "immediately" means before the document element under the tag is rendered, that is script , not waiting for subsequent loading of the document element, read to load and execute.

2. <script async src="script.js"></script>

   There async , the process of loading and rendering subsequent document elements will be script.js loaded and executed in parallel (asynchronously).

3. <script defer src="myscript.js"></script>

   There defer , the process of loading subsequent document elements is done script.js in parallel with the load (async), but script.js the execution is DOMContentLoaded done before the event fires after all element parsing is complete.

The difference between sync, defer, and async in JS