Discussion on: Don’t use for loop for JavaScript Arrays

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Replies for: I was using map to handle loops, but I found out that it struggles when handling asynchronous requests. Map doesn't wait till the previous request ...

In considering this comment I discovered one can use .reduce() to return promises in sequence (which I also thought cool enough to share):

function runPromiseInSequence(arr, input) {
  return arr.reduce(
    (promiseChain, currentFunction) => promiseChain.then(currentFunction),
    Promise.resolve(input)
  )
}

// promise function 1
function p1(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 5)
  })
}

// promise function 2
function p2(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 2)
  })
}

// function 3  - will be wrapped in a resolved promise by .then()
function f3(a) {
 return a * 3
}

// promise function 4
function p4(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 4)
  })
}

const promiseArr = [p1, p2, f3, p4]
runPromiseInSequence(promiseArr, 10)
  .then(console.log)   // 1200

Good thoughts, that's for leading me to that find.

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