⚙️ JavaScript Promises

? Quick Overview

A Promise is a JavaScript object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises help you write cleaner, more readable asynchronous code and avoid deeply nested callbacks.

? Key Concepts

• Asynchronous operations: Promises wrap tasks that complete later (like API calls or timers).
• Three states: pending, fulfilled, and rejected.
• Handlers: Use .then() for success and .catch() for errors.
• Chaining: You can return values from .then() and build step-by-step workflows.
• Promise utilities: Methods like Promise.all() let you work with multiple Promises together.

? Syntax and Theory

You create a Promise using the new Promise() constructor. It takes an executor function with two parameters: resolve (for success) and reject (for failure).

• resolve(value) → moves the Promise from pending to fulfilled.
• reject(reason) → moves the Promise from pending to rejected.
• .then(onFulfilled) runs when the Promise is fulfilled.
• .catch(onRejected) runs when the Promise is rejected.
• Each .then() returns a new Promise, which enables chaining.

? Code Examples

?️ Creating a Promise

This example shows how to create a Promise that may resolve or reject based on a condition.

let promise = new Promise(function(resolve, reject) {
// Do something...
let success = true;

if (success) {
resolve("Operation was successful!");
} else {
reject("There was an error.");
}
});

? Consuming a Promise

Use .then() to handle success and .catch() to handle errors.

promise.then(function(result) {
console.log(result);
}).catch(function(error) {
console.error(error);
});

✅ Simple Success Example

A Promise that always resolves successfully.

let promise = new Promise(function(resolve, reject) {
resolve("Success!");
});

promise.then(function(message) {
console.log("Message:", message);
});

❌ Rejected Promise Example

A Promise that is rejected and handled with .catch().

let promise = new Promise(function(resolve, reject) {
reject("Oops! Something went wrong.");
});

promise
.then(result => console.log(result))
.catch(error => console.error("Error:", error));

⏱️ Promise with setTimeout

This example simulates a delayed operation using setTimeout().

let loading = new Promise(function(resolve, reject) {
setTimeout(() => {
resolve("Data loaded!");
}, 2000);
});

loading.then(result => console.log(result));

? Chaining Promises

Each .then() can transform the value and pass it to the next .then().

new Promise((resolve, reject) => {
setTimeout(() => resolve(5), 1000);
})
.then(result => {
console.log(result); // 5
return result * 2;
})
.then(result => {
console.log(result); // 10
return result * 2;
});

? Using Promise.all()

Promise.all() runs multiple Promises in parallel and waits for all to finish.

let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = Promise.resolve(3);

Promise.all([p1, p2, p3]).then(values => {
console.log(values); // [1, 2, 3]
});