<script type=webundle> should fire a load (or error) event?

[hayatoito]

The main issue is #670.

It's unclear whether <script type=webbundle> should fire a load (or error) event or not.
And if they should be fired, when should <script type=webbundle> fire them?

• When its inline JSON is successfully parsed?
• When the bundle specified in source is loaded?
• Or any other timings?

For references, the other <script type=>-ish elements' behaviors are:

• Neither <script type=importmaps> nor <script type=speculationrules> fires a load event for an inline rule.

• Bundle Preloading doesn't mention when to fire a load event yet?
  I think it is non-trivial because it fetches a bundle many times in its life time.

We might want to align the event semantics of <script type=webbundle> with Bundle Preloading so that we can converge them in the future.

[hayatoito]

cc: @felipeerias

[hayatoito]

As a workaround, until we can have a consensus, users can try to fetch a (small) subresource immediately after inserting the script element into a document.

For example:

const script = document.createElement("script");
script.type = "webbundle";
script.textContext = JSON.stringify({ .... });
document.body.append(script);
fetch(a_tiny_subresource_in_bundle).then((_) => {
  // Alternative of a load event listener of a script element
}).catch((error) => {
  // Alternative of an error event listener of a script element
})

I'm not 100% sure whether this workaround is always applicable, however, this is fine in most cases.

Any ideas or opinions are welcome!

[cjtenny]

One thing I'm a little confused about, what do you mean by 'it fetches a bundle many times in its lifetime'? Our intention was that each bundlepreload section would result in a single request with a Bundle-Preload header; there would be no additional Bundle-Preload requests from the script element. After the page has sent a Bundle-Preload request, regular, non-<script type=bundlepreload> requests for the specified resources will then be served by the browser from the received bundle - blocking until it is available.

To review:

• Bundlepreload sections are intended as a prefetch, specifying that future requests to those resources will be served by the browser from the bundle response.
• A bundle response might not contain all the requested subresources, or might contain 404s, 500s, etc for some of them.
• A bundle response might contain additional subresources

In my mind, then, it might make sense to send:

• an error event if the JSON is not valid, as no request will be sent
• a load event for when the bundle response conforming to the header is fully received and processed, as for a JS script with an external source
• an error event if the received response doesn't have all the resources specified in the Bundle-Preload header
• an error event if the received response has additional resources specified in the Bundle-Preload header, allowing the page to detect that the server may not have understood the request, served the entire bundle as a file, etc.

The load event would be separate from the load events sent by later references to the bundled resources, which would not be sent from the script element.

What do you think? Does that match the behavior of other script type elements? I'm not familiar with speculation rules, but I'll take a look.

[cjtenny]

As a workaround, until we can have a consensus, users can try to fetch a (small) subresource immediately after inserting the script element into a document.
...
I'm not 100% sure whether this workaround is always applicable, however, this is fine in most cases.

If resources can be served from the bundled response before the whole bundle has been received, then it would be possible for this to fire a load event but the bundle itself to fire an error event - I don't know if that's good or bad. Otherwise, I think it would be a perfect workaround. Perhaps another subresource with less overhead than a script could be a little faster and more accurate for timing, but it probably doesn't matter much.

[hiroshige-g]

Basically, <script> load and error events indicate successful/failing resource loading, while parse errors go to Window's error events (e.g. window.onerror).
Inline <script type=webbundle> can trigger resource loading, so it might make sense to fire load/error events based on whether the loading of webbundle files was successful.
Inline <script type=importmap> doesn't fire load/error events, because inline import maps itself doesn't trigger resource loading.
Similar discussion for firing load events for inline <script type=module> is at whatwg/html#6421.

[cjtenny]

Got it, thank you for clarifying! It seems then like the first event I proposed should go to window.onerror, and the other 3 make sense for the script events. Does that sound reasonable? Are there any implications to that which could cause problems?

[hayatoito]

Thanks for the comments! It seems what we want to do is:

• JSON parsing error => windown.onerror
• bundle loading finished successfully => script.onload
• bundle loading failed => script.onerror

This sounds reasonable to me at first glance. At least, this would be consistent with the direction at whatwg/html#6421.

@hiroshige-g
If there are any implications which could cause problems, please let us know that.

[cjtenny]

Yeah, and I suppose defining "bundle loading failed" sub-cases is still a bit up in the air / might depend on bundle-preloading semantics. We can explore further what that is separately from the question of whether to fire the event, what do you think?

[hayatoito]

Yeah, and I suppose defining "bundle loading failed" sub-cases is still a bit up in the air / might depend on bundle-preloading semantics. We can explore further what that is separately from the question of whether to fire the event, what do you think?

Agreed. I guess defining "bundle loading failed" might be tricky on the bundle-preloading case. So we can explore that further as another issue.

[hiroshige-g]

What should occur when:

1. The first <script type=webbundle> was added with source = foo.wbn.
2. The loading of foo.wbn succeeded or failed.
3. Then the first element was replaced with the second <script type=webbundle> with source = foo.wbn.

IIUC Step 3 doesn't trigger new network request to foo.wbn. (This is intentional for successful cases. Also good for failed cases?)
Should the load/error event be fired at Step 3 for the second element?

[hayatoito]

A good question.

For those who are not familiar with the context, see the requirement 2 of <script type=webbundle> design doc, which might be helpful to understand the context more.

IIUC Step 3 doesn't trigger new network request to foo.wbn. (This is intentional for successful cases. Also good for failed cases?)

Yeah, step 3 doesn't trigger new network request. We've not decided the behavior for failed cases. Let's assume step 3 doesn't trigger new network request for a failed case too.

Given this assumption,

Should the load/error event be fired at Step 3 for the second element?

I don't have a strong preference. The behaviors of the Step3 would be one of the followings?

	1st one fired a load	1st one fired an error
A	fire a load	fire an error
B	fire nothing	fire nothing
C	fire a load	fire nothing
D	fire nothing	fire an error

Either A or B looks consistent to me. I am slightly in favor of A if we consider "re-using" is just an internal optimization and want to minimize the visible side effect.

[irori]

+1 to A.

I don't like C and D. From the user's point of view, it is difficult to use if it's not guaranteed that either load or error will be fired.

[hiroshige-g]

+1 to A. It is consistent with other cases where we fire load (and error) events without any fetch requests due to caching in the HTML spec layer:

• <img> fires load events when it hits #list-of-available-images
• For Should module scripts always fire load events? whatwg/html#6421 I expect we would fire load/error events for inline <script type=module> even if all submodules are served from module maps without new fetch requests.

Also it would keep the invariant of one load or error event for every <script type=webbundle>, which might be easier to understand for users and web developers.

We've not decided the behavior for failed cases.

Yeah, whether we reuse the failed cases or not is a separate issue to be determined.

[cjtenny]

What should occur when:

1. The first `<script type=webbundle>` was added with source = `foo.wbn`.

2. The loading of `foo.wbn` succeeded or failed.

3. Then the first element was replaced with the second `<script type=webbundle>` with source = `foo.wbn`.

IIUC Step 3 doesn't trigger new network request to foo.wbn. (This is intentional for successful cases. Also good for failed cases?) Should the load/error event be fired at Step 3 for the second element?

For bundle-preloading, step 3 depends on the contents of the element. If the resources specification contains new resources, a new request would be issued and a new load event should be fired. For consistency, I'm uncertain whether it'd be better to fire an event or not if no new resources are requested.

Sorry for the delay, have been busy with other things - will be around more starting in ~two more weeks. @felipeerias can clarify further while I'm out!

[hiroshige-g]

My spec PR #721 fires load/error events for every <script type=webbundle>, which is consistent with the comments here and corresponds to A in #697 (comment).