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Short answer

keyof T gives a union of the property keys of T. typeof x, used in a type position, gives the type of a variable. Together they derive types from existing code instead of repeating it. [S1][S2]

Example

type Point = { x: number; y: number };
type P = keyof Point;             // "x" | "y"
const cfg = { port: 80, host: "a" };
type Cfg = typeof cfg;           // { port: number; host: string }
type Key = keyof typeof cfg;     // "port" | "host"

ReturnType<typeof fn> is a common pattern: pass a function's type, not its name. [S2]

Claims

• id: C1 claim: keyof produces a union of an object type's keys. source: S1 evidence: "produces a string or numeric literal union of its keys"
• id: C2 claim: keyof is especially useful with mapped types. source: S1 evidence: "especially useful when combined with mapped types"
• id: C3 claim: typeof can be used in a type context. source: S2 evidence: "in a type context to refer to the type of a variable or property"
• id: C4 claim: ReturnType takes a function type and yields its return type. source: S2 evidence: "It takes a function type and produces its return type"

Sources

• [S1] TypeScript: Documentation - Keyof Type Operator. Microsoft (TypeScript Handbook). https://www.typescriptlang.org/docs/handbook/2/keyof-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
• [S2] TypeScript: Documentation - Typeof Type Operator. Microsoft (TypeScript Handbook). https://www.typescriptlang.org/docs/handbook/2/typeof-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_2nvk9fbp,
  author = {{iinventedlight}},
  title = {{TypeScript keyof and typeof type operators}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-keyof-typeof/},
  note = {Permanent ID iil:note:2nvk9fbp. License: CC-BY-4.0.},
}

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