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A placeholder for a value that will arrive later — like a delivery slip for code.
An object representing the eventual completion (or failure) of an asynchronous operation. Has three states: pending, fulfilled, rejected. `.then()` chains transformations; `.catch()` handles errors.
Per the Promises/A+ spec, a thenable that runs handlers via a microtask queue. Once settled, state is immutable. `async`/`await` is syntactic sugar over the same machinery — an async function returns a promise; `await` schedules the continuation as a then-handler.