If neither attribute is present, the script is downloaded and executed synchronously, and will halt parsing of the document until it has finished executing (default behavior). Scripts are downloaded and executed in the order they are encountered.
The defer
attribute downloads the script while the document is still parsing but waits until the document has finished parsing before executing it, equivalent to executing inside a DOMContentLoaded
event listener. defer
scripts will execute in order.
The async
attribute downloads the script during parsing the document but will pause the parser to execute the script before it has fully finished parsing. async
scripts will not necessarily execute in order.
Note: both attributes must only be used if the script has a src
attribute (i.e. not an inline script).
<script src="myscript.js"></script>
<script src="myscript.js" defer></script>
<script src="myscript.js" async></script>