[whatwg] script element onerror event

John J. Barton johnjbarton at johnjbarton.com
Sun May 29 08:08:15 PDT 2011

On 5/29/2011 5:42 AM, Mike Wilson wrote:
> Hi John,
> This event is actually already speced, see #14 "fire a simple event
> named error at the element" in:
> http://www.whatwg.org/specs/web-apps/current-work/#prepare-a-script
> (and the onerror attribute is valid for all elements)
Thanks. I stopped before I got that far. Step 12.5:

"Ifeventis not anASCII case-insensitive 
for either the string "|onload|" or the string "|onload()|", then the 
user agent must abort these steps at this point. The script is not 

Now I see I missed the *and* in the predicate for step 12, "If 
has an|event 
and a|for 

(I've not seen a "for" attribute on a script element. Is there any 
documentation on what it does?)

Step 14 is unclear or incomplete however:
" If the|src 
value is the empty string or if it could not be resolved,..."

Does this mean the error handler will be called in the case of 4XX, 5XX, 
and syntax errors?


> Best regards
> Mike Wilson
> John J. Barton wrote:
>> To allow optional JavaScript download, some widely used JavaScript
>> libraries, such as jQuery and requireJS, use script elements added to
>> the document dynamically by JavaScript. (Of course this
>> feature is also
>> used by applications directly as well).   For normal deployment this
>> approach works well in practice. At development time however,
>> or in the
>> presence of network or server problems, the approach gives poor error
>> recovery information. Fundamentally the problem is that the insertion
>> mechanism has no error return path.
>> The script element does support one event, 'onload' which fires after
>> the script has finished loading. I suggest the addition of one new
>> event, 'onerror', which fires in every other case. For examples, a
>> network error (4XX, 5XX) or JavaScript parse error would
>> trigger onerror
>> but not onload.  On the other hand, a runtime error for the
>> outer-function of the script element would trigger onload (I
>> guess), but
>> the developer can handle this with try/catch.
>> Very long load times would still have poor error recovery
>> information,
>> but developers could implement UI to signal "loading..." once
>> they know
>> they will get some update event eventually.
>> jjb

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