How does the 'in' operator narrow types? | Interview Question

Q: How does the 'in' operator narrow types?

**Target Audience:** Developers who narrow unions of object types and want to know the exact rules of `in` narrowing, including the cases that differ between the type checker and the running code. **Difficulty:** Medium **TypeScript version:** Every narrowing result and error below was checked with TypeScript 5.4.5 and `--strict`; the 4.9 change is cited from its release notes. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `tsc` (an `Equal ` line or a `// @ts-expect-error` line), and the **Verified** section quotes the real output in full. The general idea of narrowing is covered in [What is type narrowing and what are type guards?](/interview-question/what-is-type-narrowing-and-what-are-type-guards); this page is only about `in`. ## 1. Why This Even Matters — A Story First At an airport, three kinds of people walk up to the de

The problem

Question presented to candidate: "You have a Payment union where a card has cardNumber and a bank transfer has iban. How does if ('iban' in payment) narrow it, and what are the cases where the in operator surprises people?"

What a strong answer should cover:

"key" in obj keeps the union members that declare key (required or optional) in the true branch and the members that do not declare it, or declare it optional, in the false branch
An optional property keeps that member in both branches, so the property can still be undefined
Since 4.9, a key that no member declares is added to the type as Record<"key", unknown>, which makes in useful on unknown / object input
Run-time semantics: in walks the prototype chain ("toString" in {} is true) and throws on primitives; Object.hasOwn checks own properties only and does not narrow
When to prefer a discriminant property over in, and why in is the right tool for unions you do not control

Code / implementation expected: Yes — a function that narrows a union of object types with in, ideally including an optional-property case.

narrowing
in

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Solution

Target Audience: Developers who narrow unions of object types and want to know the exact rules of in narrowing, including the cases that differ between the type checker and the running code. Difficulty: Medium TypeScript version: Every narrowing result and error below was checked with TypeScript 5.4.5 and --strict; the 4.9 change is cited from its release notes.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Type claims are checked by tsc (an Equal line or a // @ts-expect-error line), and the Verified section quotes the real output in full. The general idea of narrowing is covered in What is type narrowing and what are type guards?; this page is only about in.

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9 min read · 9 sections

Run the code

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

TypeScriptNarrowing a union of object types with in

Practice

TypeScriptUnknown input, the prototype chain, hasOwn and primitives

Practice

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