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Short answer

Annotate parameters after their names and, optionally, the return type after the parameter list. A function type is written like an arrow function: (a: string) => void. Use generics when the output type depends on the input type. [S1][S2]

function first<T>(arr: T[]): T | undefined { return arr[0]; }

Key points

• (a: string) => void means one string parameter named a and no return value. [S1]
• Return types are usually inferred, including for generic functions. [S1]

Claims

• id: C1 claim: The simplest way to describe a function is a function type expression. source: S1 evidence: "The simplest way to describe a function is with a function type expression."
• id: C2 claim: Generics describe a correspondence between two values. source: S1 evidence: "generics are used when we want to describe a correspondence between two values"
• id: C3 claim: Parameter type annotations go after the parameter name. source: S2 evidence: "Parameter type annotations go after the parameter name:"
• id: C4 claim: You can add return type annotations. source: S2 evidence: "You can also add return type annotations."

Sources

• [S1] TypeScript: Documentation - More on Functions. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/functions.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_y14ve047,
  author = {{iinventedlight}},
  title = {{Typing functions in TypeScript}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-functions/},
  note = {Permanent ID iil:note:y14ve047. License: CC-BY-4.0.},
}

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