promise sequentially

promise_sequentially.js

    // https://hackernoon.com/functional-javascript-resolving-promises-sequentially-7aac18c4431e
    // https://decembersoft.com/posts/promises-in-serial-with-array-reduce/
    function delay() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          console.log('RESOLVED')
          resolve()
        }, 1000 * 1)
      })
    }

    const arr = new Array(10).fill(delay)
    const resultPromise = arr.reduce(function (prevPromise, currentPromise) {
      return prevPromise.then(prevResults => {
        return currentPromise().then(curResult => {
          return [...prevResults, curResult]
        })
      })
    }, Promise.resolve([]))

    resultPromise.then(results => {
      console.log(results)
    })