JavaScript Promises

JavaScript Promises

In JavaScript Promises is an object representing the eventual completion or failure of an asynchronous operation.
It allows handling async tasks more cleanly than callbacks.

Promise States

State	Description
Pending	Initial state, operation not completed yet
Fulfilled	Operation completed successfully
Rejected	Operation failed

Creating a Promise

Consuming a Promise

Example: Simulate Async Task

let fetchData = new Promise((resolve, reject) => {
setTimeout(() => {
let data = { name: "Vipul", age: 30 };
resolve(data); // Success
// reject("Failed to fetch data"); // Error
}, 2000);
});

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

let fetchData = new Promise((resolve, reject) => {

setTimeout(() => {

let data = { name: "Vipul", age: 30 };

resolve(data); // Success

// reject("Failed to fetch data"); // Error

}, 2000);

});

fetchData

.then(result => console.log("Data received:", result))

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

Chaining Promises

You can chain .then() to perform multiple steps sequentially:

Promise.all()

Promise.all() runs multiple promises in parallel and waits for all to complete.

Promise.race()

Promise.race() returns the result of the first promise to settle.

Best Practices

Always handle errors with .catch()
Chain promises instead of nesting callbacks
Use Promise.all() for parallel async operations
Consider using async/await for cleaner syntax

Summary Table

Concept	Method
Create Promise	`new Promise((resolve, reject) => {})`
Resolve	`resolve(value)`
Reject	`reject(error)`
Handle success	`.then(result => {})`
Handle error	`.catch(error => {})`
Run multiple	`Promise.all()`
First completed	`Promise.race()`

JavaScript Promises