Understanding Javascript Promises Pdf

Understanding Javascript Promises Pdf

6 min read Jun 18, 2024
Understanding Javascript Promises Pdf

Understanding JavaScript Promises: A Comprehensive Guide

This article will provide a detailed explanation of JavaScript Promises, including their syntax, benefits, and common use cases. We'll cover everything from basic promise creation and handling to advanced techniques like chaining and error handling.

What are Promises?

Promises are a way to handle asynchronous operations in JavaScript. They represent the eventual result of an operation that may take some time to complete. Essentially, a promise is a placeholder for a value that will be available in the future.

Why Use Promises?

Promises offer several advantages over traditional callback-based approaches for asynchronous operations:

  • Improved Code Readability: Promises help structure asynchronous code more clearly, making it easier to understand and maintain.
  • Error Handling: Promises simplify error handling, making it easier to manage potential errors during asynchronous operations.
  • Code Reusability: Promises promote code reusability, allowing you to write reusable functions that handle asynchronous operations consistently.
  • Chainable Operations: Promises can be chained together, allowing you to perform multiple asynchronous operations sequentially.

Creating Promises

Promises are created using the Promise constructor:

const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation
  setTimeout(() => {
    // If the operation succeeds, call resolve() with the result
  }, 1000); 
  • resolve(): This function is called when the asynchronous operation completes successfully, passing the result to the promise.
  • reject(): This function is called when the asynchronous operation fails, passing an error object to the promise.

Handling Promises

To handle the result or errors from a promise, you use the .then() and .catch() methods:

  .then((result) => {
    console.log(result); // Output: Success!
  .catch((error) => {
  • .then(): This method is executed when the promise is fulfilled (resolved). The result of the promise is passed as an argument to the function provided to .then().
  • .catch(): This method is executed when the promise is rejected. The error object passed to reject() is passed as an argument to the function provided to .catch().

Chaining Promises

Promises can be chained together using the .then() method. This allows you to perform multiple asynchronous operations in sequence:

function delay(ms) {
  return new Promise((resolve) => {
    setTimeout(resolve, ms);

  .then(() => {
    console.log("First delay completed.");
    return delay(2000); 
  .then(() => {
    console.log("Second delay completed.");

In this example, the second delay will only start after the first delay is completed.

Error Handling in Promise Chains

Errors in a promise chain are propagated to the next .catch() handler in the chain:

function delayWithError(ms) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (ms > 2000) {
        reject('Delay too long!');
      } else {
        resolve('Delay completed!');
    }, ms);

  .then(() => delayWithError(3000)) 
  .catch((error) => {
    console.error('Error:', error); 

This code will print 'Error: Delay too long!' to the console because the second delay throws an error.

Common Promise Use Cases

Promises are widely used in various scenarios:

  • Network Requests: Promises are commonly used to handle network requests (e.g., fetching data from an API).
  • File Operations: Promises simplify asynchronous file system operations, such as reading and writing files.
  • Timers: Promises are useful for creating timed operations, such as delaying execution or setting up intervals.
  • Event Handlers: Promises can be used to manage events, such as user interactions or browser events.


JavaScript Promises provide a powerful and efficient way to handle asynchronous operations. Understanding Promises is crucial for writing clean, maintainable, and robust JavaScript code that effectively deals with asynchronous processes.