NodeJS - Kill promise chain if event received

Question

I have series of promise chains, which took sufficient time to get completed. Below is the sample chain setup:

myJob1()
.then(myJob2)
.then(myJob3)
.then(myJob4)
.then(myJob5)
.then(myJob6)
.catch(myJobError);

In mean time when this job is running, if the person on UI think to cancel it, How can it be cancelled in whatever stage/function execution it is?

What can be the possible solution?

Really need more info, eg. especially when you say client. Does your client have some form of RPC in place?. Cancelling a promise chain would be easy, but it's getting the command to the back end that's the tricky part. — Keith, Aug 21 '17 at 12:03
Possible duplicate of [How to cancel an EMCAScript6 (vanilla JavaScript) promise chain](https://stackoverflow.com/questions/29478751/how-to-cancel-an-emcascript6-vanilla-javascript-promise-chain) — TheCog19, Aug 21 '17 at 12:04
You can't cancel, but you can do a reject. Which would then stop the rest of the then's from being called. You would have to test for something in each job, and if that test passed you would return a new rejected promise, ie `return Promise.reject('reason')` — Patrick Evans, Aug 21 '17 at 12:05

score 2 · Answer 1 · answered Aug 21 '17 at 13:27

One alternative to modifying code for multiple job functions might be to check a user cancelled flag between jobs. If the granularity of this kind of checking is not too course, then you could asynchronously set a (somewhat) global cancelled flag and proceed along the lines of:

let userCancelled = false;
let checkCancel = function( data) {
    if( userCancelled)
        throw new Error( "cancelled by user"); // invoke catch handling
    return data; // pass through the data
}

myJob1()
 .then(myJob2).then( checkCancel)
 .then(myJob3).then( checkCancel)
 .then(myJob4).then( checkCancel)
 .then(myJob5).then( checkCancel)
 .then(myJob6).then( checkCancel)
 .catch(myJobError);

Don't forget that if you do check the cancelled flag inside a job, all you need to do is throw an error to have it bubble down the promise chain.

Artem · Answer 2 · 2017-08-21T12:24:29.320

There is no way to cancel the promise (remember each of thens is returning a new promise) or clear the then callback.

Probably you are looking for something like redux-observable, where you can specify clause, until promise execution is actual.

See more in details: https://github.com/redux-observable/redux-observable/blob/master/docs/recipes/Cancellation.md

As alternative I may only suggest you to create and manage some flag which determines whether further process is needed or not:

// Inside each of promises in chain
if (notCancelled) {
    callAjax(params).then(resolve);
}

Or reject:

// Inside each of promises in chain
if (cancelled) {
    // Will stop execution of promise chain
    return reject(new Error('Cancelled by user'));
}

NodeJS - Kill promise chain if event received

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