For classic scripts, if the async attribute is present, then the classic script will be fetched in parallel to parsing and evaluated as soon as it is available.
For module scripts, if the async attribute is present then the scripts and all their dependencies will be fetched in parallel to parsing and evaluated as soon as they are available.
This is a boolean attribute: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.
This Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded.
Scripts with the defer attribute will prevent the DOMContentLoaded event from firing until the script has loaded and finished evaluating.
The defer attribute has no effect on module scripts — they defer by default.
This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse. async has a similar effect in this case.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script