How does Promise work in JavaScript?

• An object representing a possible final completion (or failure) of an asynchronous operation and its resulting value.
• Promise states:
  • pending: initial state, neither fulfilled nor rejected.
  • fulfilled: indicates that the operation completed successfully.
  • rejected: indicates that the operation failed.
• Transition from pending to fulfilled or rejected occurs once and is irreversible.
• Methods:
  • .then(onFulfilled, onRejected): Takes two callback functions. The first (onFulfilled) is called upon success, the second (onRejected) upon failure. Both arguments are optional. Returns a new promise.
  • .catch(onRejected): Short for .then(null, onRejected). Used for error handling. Returns a new promise.
  • .finally(onFinally): Called regardless of the promise's outcome (success or failure). Useful for resource cleanup. Returns a new promise.
• Creating a promise:

new Promise((resolve, reject) => {
  // Asynchronous operation
  const success = true;
  if (success) {
    resolve("Operation successful!"); // Transition to fulfilled
  } else {
    reject("Something went wrong..."); // Transition to rejected
  }
});

• Promise chains: .then(), .catch(), .finally() return promises, allowing chaining for sequential asynchronous operations.

fetch(url)
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));