Description
Before You File a Proposal Please Confirm You Have Done The Following...
- I have searched for related issues and found none that match my proposal.
- I have searched the current rule list and found no rules that match my proposal.
- I have read the FAQ and my problem is not listed.
My proposal is suitable for this project
- I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).
Link to the rule's documentation
https://typescript-eslint.io/rules/prefer-promise-reject-errors/
Description
When performing more complex async operations, a developer may call the reject
executor of a Promise
to propagate an AbortSignal
reason, a value obtained by a catch
block, or the reason parameter of a .then(_, onrejected)
or .catch()
callback.
in this case, it's often desirable to reject with the exact reason, to accurately propagate failures without changing them.
but, prefer-promise-reject-errors
may complain unless these values are consistently verified.
it makes sense that some users of this rule may want the rule to enforce checking, assertion, or wrapping of reasons here, but i think it's common and good practice in many cases to simply propagate reasons without modifying them.
the rule is also applied inconsistently, and a few examples are provided.
Fail
// the rule must be disabled D: this should be unnecessary
function rejectOnSignal(signal: AbortSignal) {
return new Promise<never>((_, reject) => {
// eslint-disable-next-line @typescript-eslint/prefer-promise-reject-errors -- rethrow
signal.addEventListener('abort', () => reject(signal.reason));
});
}
// the rule must be disabled D: this should be unnecessary
function rerejectOnCatch(param: Promise<unknown>) {
return new Promise<never>((_, reject) => {
// eslint-disable-next-line @typescript-eslint/prefer-promise-reject-errors -- rethrow
param.catch(e => reject(e));
});
}
Pass
// yay the rule allows rethrow :D it's not necessary to disable it
function rejectOnSignal(signal: AbortSignal) {
return new Promise<never>((_, reject) => {
signal.addEventListener('abort', () => reject(signal.reason));
});
}
// yay the rule allows rethrow :D it's not necessary to disable it
function rerejectOnCatch(param: Promise<unknown>) {
return new Promise<never>((_, reject) => {
param.catch(e => reject(e));
});
}
Additional Info
the signal example is surprisingly common - an event listener is the most certain and reliable way to immediately react to signal activation.
the promise catch/rethrow is even more contrived, and debatable, but it makes sense to me that it should be possible to propagate a 'caught' value with no intervention.
inconsistent application: things that are presently permitted, but seem to violate the rule
rethrow behavior may be achieved a couple other ways, which the rule doesn't complain about, but i think that's a problem (the rule is inconsistent) rather than an acceptable workaround.
for example, this is permitted without complaint, but it appears to mostly be a failure to understand the reject
of Promise.withResolvers
as a promise executor.
function rejectOnSignal(signal: AbortSignal) {
const { promise, reject } = Promise.withResolvers<never>();
signal.addEventListener('abort', () => reject(signal.reason));
return promise;
}
this triggers no complaints. it's kind of strange and i would also consider failure to detect this a lower priority, like the handler assignment. notably, this technique was the only way to obtain promise executor callbacks outside of the promise executor scope, before Promise.withResolvers
became available.
export function rejectOnSignal(signal: AbortSignal) {
let rejectExecutor;
const promise = new Promise<never>((_, reject) => {
rejectExecutor = reject;
});
signal.addEventListener('abort', () => rejectExecutor(signal.reason));
return promise;
}
this next example is incorrect for a few reasons (the signal itself becomes the propagated value, this may clobber existing handlers), this is permitted without complaint. this appears mostly to be a failure to understand that assigning reject
to an event handler attribute will result in a call to reject
.
function rejectOnSignal(signal: AbortSignal) {
return new Promise<never>((_, reject) => {
signal.onabort = reject;
});
}