What does the satisfies operator do? | Interview Question

Q: What does the satisfies operator do?

**Target Audience:** Developers who know type annotations and `as`, and want to know what `satisfies` adds and when to use it. **Difficulty:** Medium **TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `satisfies` arrived in TypeScript 4.9 ([Announcing TypeScript 4.9](https://devblogs.microsoft.com/typescript/announcing-typescript-4-9/), November 2022). > **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` (a `// @ts-expect-error` line fails the build if the error does not happen), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First An airline has a rule: a carry-on must fit the 55 × 40 × 20 cm frame at the gate. - **Option A:** the agent checks your bag in the frame, and then writes "standard carry-on" on the tag and forgets everything else about it. Later, nobo

The problem

Question presented to candidate: "What does the satisfies operator do in TypeScript? How is const x = value satisfies T different from const x: T = value and from value as T?"

What a strong answer should cover:

expr satisfies T checks that the expression is assignable to T (including excess-property and missing-property checks) but the expression keeps its own inferred type
Compared with an annotation: both check, but the annotation replaces the type with T, losing per-property detail (a string | RGB value stays a union)
Compared with as: an assertion barely checks (a missing key slips through), satisfies reports the error at the offending property
It provides a contextual type, so callback parameters inside the object are typed and tuple targets produce tuples
It is an expression operator: usable in returns, arguments and exports, not only on variable declarations; it is erased from the JavaScript
Introduced in TypeScript 4.9 (November 2022)

Code / implementation expected: Yes — the same object written with an annotation, with as, and with satisfies, showing one compile error each would or would not catch.

satisfies

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Solution

Target Audience: Developers who know type annotations and as, and want to know what satisfies adds and when to use it. Difficulty: Medium TypeScript version: Checked with TypeScript 5.4.5 and --strict. satisfies arrived in TypeScript 4.9 (Announcing TypeScript 4.9, November 2022).

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 (a // @ts-expect-error line fails the build if the error does not happen), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

An airline has a rule: a carry-on must fit the 55 × 40 × 20 cm frame at the gate.

Option A: the agent checks your bag in the fra

The solution is hidden so you can try first

8 min read · 9 sections

Run the code

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

TypeScriptAnnotation vs as vs satisfies on a colour palette

Practice

TypeScriptContextual typing and satisfies as an expression

Practice

Discussion

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