# Promise Error Handling Hooks: Rough Spec Algorithm

## ECMAScript/V8 algorithm

### General modifications

All promises need to get a unique promise ID of some sort, e.g. a string or number. These can be created lazily if that is easier.

All promises get a "rejection status" field, which is one of: "nothing"; "notified of rejection"; ".then'd"

### Modification to PromiseReject

If a promise is rejected (which currently always goes through PromiseReject, both in the spec and in V8), and its rejection status is not ".then'd", send a message to the host environment consisting of "potentially unhandled rejection: (the promise ID, the rejection reason)", and set its rejection status to "notified of rejection."

### Modification to Promise.prototype.then

If the promise's rejection status is "notified of rejection," send a message to the host environment consisting of "rejection handled: (the promise ID)".

Set the promise's rejection status to ".then'd".

## Host environment algorithm

The host environment has a list of outstanding rejected promise IDs and about-to-be-notified rejected promise IDs.

Receive "potentially unhandled rejection" messages, and:
 - Add the given promise ID to the list of about-to-be-notified rejected promise IDs.
 - Queue a notify-rejected task, with parameters promise ID and rejection reason.

Receive "rejection handled" messages, and:
 - Remove the given promise ID from the list of about-to-be-notified rejected promise IDs, if present.
 - If the given promise ID is in the list of outstanding rejected promise IDs, notify the user (via devtools and the onrejectionhandled event) that the rejection was handled.
 - (Note that only one of these two alternatives should ever occur.)

The notify-rejected task, which is given a promise ID and rejection reason, does the following:
 - If the promise ID is no longer in the list of about-to-be-notified rejected promise IDs, do nothing.
 - Otherwise,
   - Notify about a rejection with the given promise ID and rejection reason.
   - Add the given promise ID to the list of outstanding rejected promise IDs.