Javascript Promise Lifecycle

• Javascript

• Javascript Basic

• • Javascript Basics

  • Array

  • Array Extension

  • Class

  • Data Types

  • Date

  • Date Extension

  • Function

  • Generator

  • Iterator

  • Json

  • Language

  • Math

  • Math Extension

  • Number Extension

  • Object

  • Object Extension

  • Operators

  • Promise

  • Regular Expressions

  • Set

  • Statements

  • String

  • String Extension

  • Javascript Browser

  • BOM

  • DOM

  • Storage

  • Javascript Usage

  • Algorithm

  • Data Structure

  • Interview Question

Introduction

Each promise goes through a short lifecycle starting in the pending state.

The pending state indicates that the asynchronous operation hasn't completed yet.

A pending promise is considered unsettled.

In the following code, the promise is in the pending state as soon as the readFile() function returns it.

let promise = readFile("example.txt");

Once the asynchronous operation completes, the promise is considered settled.

It will enter one of two possible states:

• Fulfilled - The promise's asynchronous operation has completed successfully.
• Rejected - The promise's asynchronous operation didn't complete successfully due to either an error or some other cause.

An internal PromiseState property is set to pending, fulfilled, or rejected to reflect the promise's state.

This property isn't exposed on promise objects, so you can't determine which state the promise is in programmatically.

We can take a specific action when a promise changes state by using the then() method.

The then() method takes two arguments.

Related

• Javascript Spread Operator pass function parameters
• Javascript Promise
• Javascript Promise catch()
• Javascript Promise reject()
• Javascript Promise resolve()