Promise Object in JavaScript

Overview

• Promise: Represents eventual completion or failure of an asynchronous operation.
• States:
  • Pending: Initial state, neither fulfilled nor rejected.
  • Fulfilled: Operation completed successfully.
  • Rejected: Operation failed.
• Resolved: Promise is settled to match the eventual state of another promise.

Using Promises

• Promises allow asynchronous methods to return values like synchronous methods.
• Promise handlers (then, catch, finally) are triggered based on the promise state.
• Chained Promises: Allows further actions to be associated with the settled promise.
  • then(): Takes two arguments; handlers for fulfilled and rejected cases.
  • catch(): Handles promise rejections.
  • finally(): Executes when promise is settled, regardless of outcome.

Promise Methods

• Static Methods:
  • Promise.all(): Fulfills when all promises are fulfilled, rejects if any are rejected.
  • Promise.allSettled(): Fulfills when all promises settle.
  • Promise.any(): Fulfills when any promise fulfills, rejects if all reject.
  • Promise.race(): Settles when any promise settles.
  • Promise.resolve(): Returns a fulfilled promise.
  • Promise.reject(): Returns a rejected promise.
• Instance Methods:
  • Promise.prototype.then(): Appends fulfillment and rejection handlers.
  • Promise.prototype.catch(): Appends a rejection handler.
  • Promise.prototype.finally(): Appends a handler for when promise is settled.

Concepts

• Thenables: Objects implementing the then method, allowing them to be used like promises.
• Promise Concurrency: Promises can be executed concurrently using methods like Promise.all.
• Job Queue: JavaScript executes promises by adding them to a job queue, ensuring asynchronous execution.

Examples

• Basic Example: Promise with setTimeout to simulate async behavior.
• Diverse Situations: Handling errors and chaining promises.
• Advanced Example: Demonstrates promise creation and fulfillment logging.
• XHR Loading: Using promises to handle image loading with XMLHttpRequest.

Additional Topics

• Incumbent Settings Object: Ensures the correct environment is used for executing JavaScript code in specific contexts.

Specifications and Compatibility

• ECMAScript: Defined under ECMAScript 2026 Language Specification.

These notes cover the key concepts and methods related to JavaScript Promises, providing a foundational understanding of how Promises work and how they can be used to handle asynchronous operations efficiently.