How to execute promises sequentially, passing the parameters from an array?

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You can make the repeated application of .then into a fold pretty neatly if you’re okay with creating as many promises as array elements as is the case in the question:

myArray.reduce(
  (p, x) =>
    p.then(_ => myPromise(x)),
  Promise.resolve()
)

but an async function, for example, doesn’t require that:

const mapSeries = async (iterable, action) => {
  for (const x of iterable) {
    await action(x)
  }
}

mapSeries(myArray, myPromise)

which is built into the excellent promise library Bluebird as mapSeries:

Promise.mapSeries(myArray, myPromise)

Runnable snippet:

const myArray = [1, 2, 3, 4, 5, 6]

const sleep = ms =>
  new Promise(res => {
    setTimeout(res, ms)
  })

const myPromise = num =>
  sleep(500).then(() => {
    console.log('done: ' + num)
  })

myArray.reduce(
  (p, x) =>
    p.then(_ => myPromise(x)),
  Promise.resolve()
)

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