What is a Promise and what are its states? | Interview Question

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

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Understand 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

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Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

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Study the 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 M

Solution ready — 2 min read

Classified // press E to declassify

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Run the code

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

Practice

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