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Script

The <script> tag.

Defer​

A Boolean attribute that is set to indicate a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded.

info

Scripts with the defer attribute will execute in the order in which they appear in the document.

Usage​

In practice, defer is used for scripts that need the whole DOM and /or their relative execution order is important.

Async​

async scripts load in the background and run when ready.

So an async script is completely independent:

  • DOMContentLoaded and async scripts don't wait for each other.
  • Other scripts don't wait for async scripts, and async don't wait for them.

Usage​

The async attribute is used for independent scripts, like counters or ads. And their relative execution order does not matter.

References​