JavaScript Promises

JavaScript promises are a feature in the language used to handle asynchronous operations more effectively and avoid callback hell (nested callbacks). A promise represents a value that may be available now, in the future, or never. It acts as a placeholder for a result that will be returned later, allowing you to write cleaner and more manageable asynchronous code.

States of a Promise

1. Pending: The initial state; the promise is neither fulfilled nor rejected.

2. Fulfilled: The operation is completed successfully, and the promise has a resulting value.

3. Rejected: The operation failed, and the promise has a reason for the failure (an error).

Using Promises

A promise is created using the promise constructor:


let promise = new Promise((resolve, reject) => {
    // Perform an operation
    let success = true; // Example condition
    if (success) {
        resolve("Operation was successful!");
    } else {
        reject("Operation failed.");
    }
});

You can handle the resolved or rejected outcomes using .then() and .catch():


promise.then((result) => {
        console.log(result); // "Operation was successful!"
    }).catch((error) => {
        console.error(error); // "Operation failed."
    });

Chaining Promises

Promises can be chained to handle sequences of asynchronous operations:


fetch("https://api.example.com/data")
    .then((response) => response.json())
    .then((data) => console.log(data))
    .catch((error) => console.error("Error:", error));