A test to see if you can truly cancel JS promises. Spoiler: you can't.

can-js-promises-be-cancelled.js

// Question: does calling reject on a running promise cut it short?
// Answer: sort of. It returns to the caller early but it keep running behind
//         the scenes. So more of a short-circuit than really being cancelled.

const printFrequency = 500
const printRounds = 10
const cancelPoint = 7

let someOtherState = 'untouched'

const { thePromise, cancel } = (function slowCancellablePromise() {
  let cancel
  const thePromise = new Promise((resolve, reject) => {
    cancel = () => {
      reject('cancelled')
    }
    let count = 1
    function printAndWait() {
      console.log('hello ' + count)
      someOtherState = count
      setTimeout(() => {
        if (count++ >= printRounds) {
          return resolve()
        }
        printAndWait()
      }, printFrequency)
    }
    printAndWait()
  })
  return { thePromise, cancel }
})()

thePromise
  .then(() => console.log('finished normally'))
  .catch(err => console.error('finished with error: ' + err))
  .finally(() => {
    console.log(
      `after promise has returned, someOtherState = ${someOtherState}`,
    )
    console.log('waiting until the promise has actually finished...')
    setTimeout(() => {
      console.log(
        `after promise has actually finished, someOtherState = ${someOtherState}`,
      )
    }, printFrequency * (printRounds - cancelPoint))
  })

console.log('waiting for a bit...')
setTimeout(() => {
  console.log('cancelling promise')
  cancel()
  console.log('the promise should stop printing')
}, printFrequency * cancelPoint)

// the output:
//   hello 1
//   waiting for a bit...
//   hello 2
//   hello 3
//   hello 4
//   hello 5
//   hello 6
//   hello 7
//   cancelling promise
//   the promise should stop printing
//   finished with error: cancelled
//   after promise has returned, someOtherState = 7
//   waiting until the promise has actually finished...
//   hello 8
//   hello 9
//   hello 10
//   after promise has actually finished, someOtherState = 10