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Answer: The new satisfies operator lets us validate that the type of an expression matches some type, without changing the resulting type of that expression. (S1)

Example

type Colors = "red" | "green" | "blue";
type RGB = [number, number, number];

const palette = {
  red: [255, 0, 0],
  green: "#00ff00",
  blue: [0, 0, 255],
} satisfies Record<Colors, string | RGB>;

palette.green.toUpperCase(); // OK: still known to be a string
palette.red.at(0);            // OK: still a tuple

Annotation vs satisfies vs as

| Form | Checks? | Resulting type | |---|---|---| | const x: T = v | yes | T | | v satisfies T | yes | inferred type of v |

Annotations go after the thing being typed.

Claims

• id: C1 claim: satisfies validates a type without changing the resulting type. source: S1 evidence: "The new satisfies operator lets us validate that the type of an expression matches some"
• id: C2 claim: The release notes use satisfies with Record of Colors. source: S1 evidence: "} satisfies Record<Colors, string | RGB>;"
• id: C3 claim: Type annotations always go after the thing being typed. source: S2 evidence: "Type annotations will always go after the thing being typed."
• id: C4 claim: Variables can have an optional type annotation. source: S2 evidence: "you can optionally add a type annotation to explicitly specify the type of the variable:"

Sources

• [S1] TypeScript: Documentation - TypeScript 4.9. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
• [S2] TypeScript: Documentation - Everyday Types. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/everyday-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_1xbkjv9h,
  author = {{iinventedlight}},
  title = {{The satisfies operator in TypeScript}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-satisfies-operator/},
  note = {Permanent ID iil:note:1xbkjv9h. License: CC-BY-4.0.},
}

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