41. What is a Promise?

What is a Promise?

A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It allows you to handle asynchronous operations more easily compared to callbacks, making the code cleaner and easier to manage.

Why Do You Need a Promise?

1. Avoid Callback Hell: Promises prevent deeply nested callbacks (also known as "callback hell") by chaining operations.
2. Better Error Handling: Promises provide a cleaner way to catch errors using .catch() instead of handling them at each callback level.
3. Improved Code Readability: Promises allow chaining using .then(), making the code easier to follow.

Three States of a Promise

A Promise can have three states:

1. Pending – Initial state; the promise is neither fulfilled nor rejected.
2. Fulfilled – The operation completed successfully, and the promise has a resulting value.
3. Rejected – The operation failed, and the promise has a reason for the failure.

Example:

Here's an example of a Promise:

javascript
CopyEdit
const fetchData = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            const success = true; // Change to false to simulate an error
            if (success) {
                resolve("Data fetched successfully");
            } else {
                reject("Failed to fetch data");
            }
        }, 2000);
    });
};

fetchData()
    .then((result) => {
        console.log("Success:", result); // Output if resolved
    })
    .catch((error) => {
        console.log("Error:", error); // Output if rejected
    });

Explanation:

1. When fetchData() is called:
   • The promise starts in the pending state.
2. After 2 seconds:
   • If success = true, the promise moves to the fulfilled state, and resolve() returns the result.
   • If success = false, the promise moves to the rejected state, and reject() returns the error message.
3. .then() handles the successful response.
4. .catch() handles the error case.

This example demonstrates how Promises simplify asynchronous operations and make error handling more manageable.