Sequential Execution of Promise based Tasks

Execute an array of promises in sequence, one after another:

let tasks = [ /*promise-based tasks*/ ]
let promise = Promise.resolve()
tasks.forEach(function(task) {
  promise = promise.then(function() {
    return task()
  })
})
promise.then(function() {
  // Do something after all tasks completed
})


// Alternative with reduce
let tasks = [ /*promise-based tasks*/ ]
let promise = tasks.reduce(function(prev, task) {
  return prev.then(function() {
    return task()
  })
}, Promise.resolve())
promise.then(function() {
  // Do something after all tasks completed
})

What’s happening:

• Promise.resolve() creates a promise that resolves without any conditions simply to begin the promise chain
• The forEach loop calls then() on the previous promise in each iteration to create a promise chain
• At the very end, the callback within the final then() function triggers when the last promise-based task completes.

Posted on: Fri, 01 Mar 2019 06:21:11 GMT

Tags:

javascript

design pattern