serial.js

function promiseSerial(tasks) {
  finalTaskPromise = tasks.reduce(function (prevTaskPromise, task) {
    return prevTaskPromise.then(task)
  }, Promise.resolve())

  return finalTaskPromise
}

const tasks = [
() => new Promise((resolve) => setTimeout(() => {console.log(1); resolve(1)},1000)),
() => new Promise((resolve) => setTimeout(() => {console.log(2); resolve(2)},2000)),
() => new Promise((resolve) => setTimeout(() => {console.log(3); resolve(3)},3000)),
]

promiseSerial(tasks)