What is a Promise in JavaScript?

Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. A Promise is in one of three states: pending, fulfilled, or rejected. It allows avoiding "callback hell" and makes asynchronous code more readable and manageable.

Main methods of Promise:

• .then(): Handles the successful completion of a Promise and returns a new Promise. It takes a handler function for the successful result.
• .catch(): Handles the rejection of a Promise and returns a new Promise. It takes a handler function for errors. It is syntactic sugar for .then(null, rejectionHandler).
• .finally(): Executes a function regardless of the Promise's outcome (success or failure). Useful for resource cleanup.
• Promise.all(iterable): Returns a new Promise that resolves when all Promises in the provided iterable are resolved. It rejects if any of the Promises reject.
• Promise.race(iterable): Returns a new Promise that resolves or rejects as soon as any of the Promises in the provided iterable resolve or reject.
• Promise.resolve(value): Returns a Promise that resolves with the specified value.
• Promise.reject(reason): Returns a Promise that rejects with the specified reason.

Example usage:

// Creating a new Promise
const myPromise = new Promise((resolve, reject) => {
  const success = true; // Simulating a successful or unsuccessful operation

  if (success) {
    setTimeout(() => {
      resolve("Operation completed successfully!"); // Call resolve on success
    }, 1000);
  } else {
    setTimeout(() => {
      reject("Operation failed!"); // Call reject on failure
    }, 1000);
  }
});

// Using the Promise
myPromise
  .then((result) => {
    // This block executes on successful Promise resolution
    console.log(result);
    return "Processed!"; // Returning a value for the next .then
  })
  .then((nextResult) => {
    // This block executes after the previous .then
    console.log(nextResult);
  })
  .catch((error) => {
    // This block executes if the Promise is rejected
    console.error(error);
  })
  .finally(() => {
    // This block executes always, regardless of the outcome
    console.log("Promise completed.");
  });