What is a Promise? Explain its states and how chaining works.

A Promise is an object that stands for the future result of an async operation. Think of a restaurant buzzer: you get it now, and it tells you later if your food is ready or if something went wrong.

A promise has three states:

• pending: the work is not finished yet. Every promise starts here.
• fulfilled: the work succeeded, with a value.
• rejected: the work failed, with a reason.

A promise settles (becomes fulfilled or rejected) only once. After that it never changes.

How chaining works:

• then(onFulfilled, onRejected) registers callbacks. It always returns a new promise.
• The new promise resolves with whatever the callback returns.
• If the callback returns a promise, the chain waits for that promise.
• If the callback throws, the new promise rejects.

This is what makes flat chains possible, instead of callbacks nested inside callbacks.

A rejection skips then handlers until it reaches a catch, or a then with a second argument. catch can recover by returning a value, and then the chain continues. finally runs on both success and failure, and passes the original result through.

Promise callbacks always run asynchronously, as microtasks.