I also looked through the WebUSB proposal, and it also doesn't contain any indications of such functionality. (Or maybe it can be achieved by just saying that the returned promise may be cancellable; however, the ECMAScript 6 or ECMAScript 2017 Promises are not cancellable.)

There's a lot of plumbing that will need to be put in place for this to work. We need to assign some sort of ID to pending transfers so that API clients can refer to them later to cancel them and make that available through the whole stack.

I'd love to use cancellable Promises in WebUSB and will probably wait for those to be available before implementing this in the WebUSB spec.

For Chrome Apps we will likely implement some sort of chrome.usb.cancelTransfer() function and the question is simply where and how to communicate the ID to the application. It may actually be better for the app to provide us with an optional ID parameter just because of the way that extensibility in Chrome Extensions APIs works.

> For Chrome Apps we will likely implement some sort of
> chrome.usb.cancelTransfer() function and the question is simply where
> and how to communicate the ID to the application. It may actually be
> better for the app to provide us with an optional ID parameter just
> because of the way that extensibility in Chrome Extensions APIs works.

SGTM.
A different approach would be to return some proxy object from the chrome.usb.*Transfer functions, that has a "cancel" method. But I'm not sure that this is supported by the Chrome Extension APIa framework (as the suggested solution is basically a combination of both synchronous and asynchronous result returning).