Here's a pass at customizable missing element warnings. I'm not attached to my approach; feel free to suggest different variable names, different attribute names, changes to the behaviour of the different modes.

I should also say that it occurred to me only as I was typing this up that perhaps this is actually the wrong approach; instead of specifically addressing missing elements, perhaps it would be better to implement a debug concept that would allow the developer to make decisions for all errors. There are lots of warnings etc in the codebase that have nothing to do with missing elements. Opinions, please!

After deliberation, I took Marco's idea to have multiple strategies and ran with it. I figure that we can always pull these extra options out if we don't want them.

CableReady.initialize now supports an onMissingElement option which sets a global value. As always, it defaults to warnings (warn) but also supports ignore, event, exception. It will warn if you pass an invalid option, falling back to warn.

CableReady.initialize({ consumer, onMissingElement: 'event' })

event raises a cable-ready:missing-element event on document. exception throws an exception.

The emitMissingElementWarnings option on perform and performAsync has been removed in favor of onMissingElement. It's a major version bump, after all. A value provided overrides the global value.

if (data.cableReady) perform(data.operations, { onMissingElement: 'exception' })

We could optionally test for emitMissingElementWarnings: false, issue a deprecation warning and set onMissingElement: 'ignore', but it's not implemented at this time. Opinions, please!

Finally, the stream_from web component checks for a missing attribute and falls back to the global setting. The WC supports warn, ignore and event, but not exception. This is because there's no way for a developer to catch an exception raised by the WC.

<%= stream_from :all_users, html_options: {missing: :ignore} %>

Again, I'm happy to change the attribute. I started with short and simple, because missing-element requires using double-quotes which I find ugly:

<%= stream_from :all_users, html_options: {"missing-element": :ignore} %>