直列Promise

promise.js

console.log('start')

var array = [1,2,3];

array.reduce((acc, current)=>{
  return acc.then(() => wait(current))
}, Promise.resolve())

function wait (x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log(x)
      resolve()
    }, 1000)
  })
}