[whatwg] The Attributes of the script element

Boris Zbarsky bzbarsky at MIT.EDU
Fri Jul 30 07:46:11 PDT 2010

On 7/30/10 4:33 AM, zhao Matt wrote:
> Obviously, the book thinks Client-side JavaScript is (or behaves as if
> it is) *single-threaded*.
> However,
> HTML5 spec states 'the user agent must immediately execute the script
> block, even if other scripts are *already* executing.',
> Does it imply that scripts can be run in parallel?

This seems to be a common misconception.

Being single-threaded doesn't mean things can't be _reentrant_.  So if 
you have a script running and that script inserts a <script> node with a 
textnode child into the document, the script contained in that textnode 
will be executed immediately even though there is already a script 
running.  This will happen before control returns to the original script 
from the appendChild call.

Whether this setup maps to "run in parallel" in your head, I don't know.

> "Running a script: When a script element is to be run, the user agent
> must act as follows:..."
> The process includes 9 steps, but I don't sure which step will take the
> attribute 'async' into consideration.

Um... Step 9, which is the only one that mentions it, no?

Having an "async" attribute prevents a script from taking the first two 
options in that step.


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