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Short answer

Mapped types build new object types by iterating keys, usually with keyof. They extend index-signature syntax and can add readonly or optional modifiers. [S1]

Sketch

type Partial<T> = { [K in keyof T]?: T[K] };

Points

• Mapped types iterate keys to create a type. [S1]
• They build on index signature syntax. [S1]
• keyof yields a literal union of an object type's keys. [S2]

Claims

• id: C1 claim: Mapped types iterate keys to create a type. source: S1 evidence: "to iterate through keys to create a type"
• id: C2 claim: Mapped types build on index signature syntax. source: S1 evidence: "Mapped types build on the syntax for index signatures"
• id: C3 claim: Mapped types support readonly and optional modifiers. source: S1 evidence: "two additional modifiers which can be applied during mapping: readonly and ?"
• id: C4 claim: keyof produces a literal union of keys. source: S2 evidence: "produces a string or numeric literal union of its keys"

Sources

• [S1] TypeScript: Documentation - Mapped Types. Microsoft (TypeScript Handbook). https://www.typescriptlang.org/docs/handbook/2/mapped-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.
• [S2] Keyof Type Operator. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/keyof-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.

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

Source notice: TypeScript Handbook / Website docs © Microsoft. Licensed under Creative Commons Attribution 4.0 International (https://github.com/microsoft/TypeScript-Website/blob/v2/LICENSE).

@misc{iil_note_b03fsfwj,
  author = {{iinventedlight}},
  title = {{TypeScript mapped types: build one type from another}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-mapped-types/},
  note = {Permanent ID iil:note:b03fsfwj. License: CC-BY-4.0.},
}

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