1. Definition

A Promise is an object in JavaScript that represents the eventual completion or failure of an asynchronous operation and its result.

2. Promise States

• Pending: Initial state
• Fulfilled: Operation completed successfully
• Rejected: Operation failed

3. Key Idea

• Handles asynchronous operations
• Avoids callback hell
• Improves readability
• Supports chaining

4. How it Works

• Create a Promise
• Perform async operation
• Resolve (success) or Reject (error)
• Handle result using .then() and .catch()

5. Example

const myPromise = new Promise((resolve, reject) => {
  let success = true;

  if (success) {
    resolve("Task completed");
  } else {
    reject("Task failed");
  }
});

myPromise
  .then(result => console.log(result))
  .catch(error => console.log(error));

6. Promise Chaining

loginUser(user)
  .then(getOrders)
  .then(getOrderDetails)
  .then(processPayment)
  .then(() => console.log("Done"))
  .catch(err => console.log(err));

7. Advantages

• Avoids callback hell
• Better readability
• Easy error handling
• Supports chaining

8. Disadvantages

• Slightly complex for beginners
• Can still become complex if overused