How does promise chaining work, and what happens if a `.then` handler returns a value vs a promise?

Short Interview Answer

Each `.then` returns a new promise; returned values fulfill it, thrown errors reject it, and returned promises adopt that promise's state.

Detailed Explanation

Chaining avoids nesting by passing results along. If you return a plain value, the next `.then` receives it. If you return a thenable/promise, the chain waits for it (assimilation). If you omit a return, the next handler gets `undefined`. Throwing or returning `Promise.reject` rejects the downstream chain until a `.catch`. Always return/throw intentionally inside handlers to keep control flow clear.

Example

Promise.resolve(1)
  .then((n) => n + 1)
  .then((n) => Promise.resolve(n * 2))
  .then(console.log); // 4

Interview Tip

Warn about missing `return` inside `.then` — common bug.

Common Mistake

Nesting `.then` inside `.then` unnecessarily (recreating callback hell).