Question

I read this nice article on how external scripts block the UI thread but it wasn’t clear to me whether the blocking is actually due to the presence of the <script> tag or the src='/myscript.js' src attribute.

My question is does inline javascript (lacking a src attribute declaration), for example this:

<script type='text/javascript'> alert('am i blocking too?');</script>

or this:

<script type='text/javascript'> var php='<?=json_encode($myObj)?>';</script>

also block the UI thread?

Answer 1

Any loading of a JS file or any execution of any JS (whether in an external file or inline) will block the UI thread.

The exception for the <script> tag is an asynchronous load where the script will load and execute asynchronously in the background.

There are also “deferred” loads (i.e. the defer attribute) which tells the browser not to actually execute the JS therein until the rest of the page has loaded.