What is a Promise and what are its states?

Q: What is a Promise and what are its states?

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every state-transition claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A verdict read aloud in a courtroom is genuinely final the moment it's spoken — the judge cannot walk back in five minutes later and announce a completely different verdict for the same case; that case is already closed. A Promise's settlement works exactly the same way: the very first time `resolve` or `reject` genuinely takes effect, the Promise's fate is permanently sealed — any later attempt to call either function again is like the judge trying to re-announce a verdict for an already-closed case: it genuinely has no effect at all. ## 2. The Core Idea 📌 **Interview term:** a Promise has exactly 3 states — pending, fulfilled, rejected. Once s

The problem

Question presented to candidate: "If a Promise's executor function calls resolve() and then, a moment later, calls reject() as well — what actually happens? Does the Promise genuinely change from fulfilled to rejected?"

What a strong answer should cover:

📌 Interview term: the three Promise states — pending (not yet settled), fulfilled (completed successfully, with a value), and rejected (failed, with a reason) — every Promise starts pending and can transition to exactly one of the other two states.
📌 Interview term: the real, direct answer to the prompt — verified directly: once a Promise is settled (fulfilled or rejected), its state is genuinely permanent — calling resolve/reject again afterward is genuinely a no-op, silently ignored. A Promise resolved first with "first value" and then immediately resolved again with a different value genuinely keeps the FIRST value — the second call has zero effect.
📌 Interview term: .then() always returns a NEW Promise — verified directly: chaining .then() genuinely never returns the same Promise object — this is exactly the real mechanism that makes Promise chaining work at all.
📌 Interview term: a thrown error inside .then() produces a rejected Promise — verified directly: throwing inside a .then() callback genuinely does NOT crash the program — it genuinely produces a rejected Promise, catchable by a .catch() further down the chain.
A precise answer names that resolving a Promise WITH another Promise (or any "thenable") genuinely flattens/unwraps it rather than nesting — verified directly, resolving with an already-resolved inner Promise genuinely produces the inner Promise's own real value, not a Promise-wrapped-in-a-Promise.

Clarifying questions expected:

None — this is a definitional/technical question; directly answering the prompt's own resolve-then-reject scenario (state permanence) is the strong signal.

Code / implementation expected: Yes — a real Promise where resolve is called twice (with a second, different value) is the clearest, most convincing demonstration of state permanence.

promises

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals/async interviews. Difficulty: Medium

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every state-transition claim below was actually run in Node.

1. Why This Even Matters — A Story First

A verdict read aloud in a courtroom is genuinely final the moment it's spoken — the judge cannot walk back in five minutes later and announce a completely different verdict for the same case; that case is already closed. A Promise's settlement works exactly the same way: the very first time resolve or reject genuinely takes effect, the Promise's fate is permanently sealed — any later attempt to call either function again is like the judge trying to re-announce a verdict for an already-closed case: it genuinely has no effect at all.

2. The Core Idea

📌 **

The solution is hidden so you can try first

6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal proof: a Promise's settlement is genuinely permanent — a second resolve() call has zero effect, and .then() always returns a new Promise