[whatwg] Proposal for separating script downloads and execution

Boris Zbarsky bzbarsky at MIT.EDU
Thu Feb 10 19:51:18 PST 2011

On 2/10/11 10:32 PM, timeless wrote:
> i believe the only requirements for:
> <script>
> function x(){....}
> function y(){....}
> </script>
> are:
> 1. if x fails to compile, y is not evaluated
> 2. y can fail to compile too
> 3. if something fails to compile, window.onerror is triggered
> 4. if x could compile, then window.x is defined to be a Function object

I don't think so.  If there is any parse or compilation or whatever you 
want to call it error, the script is never executed, so window.x is 
never defined.

Also, I would fully expect it to be a web compat requirement that 
window.onerror not be triggered until the script would be evaluated.  If 
you parse/compile/whatever before that and hit an error, you'd need to 
save that and report it at the "right" time.

Which means the that parse/compile/whatever process is currently not 
observable directly.  And that's a good thing!

> in theory, i believe a js engine could choose to discard all work it
> has done to validate syntax for x+y beyond saving those coordinates,
> and then either do the proper ast / bytecode / machine code generation
> lazily (or schedule to do it on a thread).

Sure; a js engine could also not do any syntax validation at all, until 
it needs to run the script...


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