On Mon, Apr 23, 2018 at 10:56 AM, Oliver Dunk <[email protected]> wrote: > My proposal is that we add a way of removing a particular callback, or all > callbacks, from a Promise. This is different to cancelling a Promise and > would instead happen if you want the operation to continue but are no longer > interested in running a function when the Promise is resolved or rejected. > > This would be implemented in two ways. The first I believe to be most useful, > and the second is a sort of “sub-proposal”, that I would be happy to leave > out: > > 1. A new `Promise.prototype.clear()` method. This would remove all callbacks > currently added to a Promise.
This is almost certainly a no-go - it's a huge capability leak. If Alice has attached a .then() callback to a promise, Bob shouldn't be able to cancel that without Alice's permission. > 2. A `Promise.prototype.clear(promise)` method, which takes the Promise > returned when `Promise.prototype.then()` or `Promise.prototype.catch()` was > called. This would remove that specific callback leaving any others. I don’t > know if another argument would make better sense here? Maybe an identifier to > the one used in `clearInterval()`. > > The use case that I see for this is for a Promise equivalent to > `EventTarget.removeEventListener()`. Possible, but seems unnecessary. In EventTarget, the listener is attached *forever*, because there's no limit to how many times the event can fire. Removing a listener is thus a useful feature, to prevent listeners from stacking up infinitely. For a promise, tho, the callback is one-and-done; after it resolves, the callback is released and can be collected. There's no strong memory-related reason to "cancel" a promise callback; if you need that functionality on your own, you can just have the callback close over a boolean, and check the boolean once it resolves and either continue or return immediately. This would only be a few lines to write a helper function for. As others have stated, it also makes the semantics of the returned promise from .then() unclear. A forever-pending promise is a possible resolution, but it feels a bit awkward. ~TJ _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
