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Short answer

Narrowing is how TypeScript shrinks a union after checks such as typeof, instanceof, or a custom type predicate. [S1]

Points

• typeof checks are type guards. [S1]
• In JavaScript, typeof null is "object", which surprises people. [S1]
• instanceof is also a type guard. [S1]
• Union types let you build new types from existing ones; operations must be valid for every member. [S2]

Claims

• id: C1 claim: typeof checks act as type guards. source: S1 evidence: "checking against the value returned by typeof is a type guard"
• id: C2 claim: typeof null is object in JavaScript. source: S1 evidence: "typeof null is actually "object""
• id: C3 claim: instanceof is a type guard. source: S1 evidence: "instanceof is also a type guard"
• id: C4 claim: Operations on a union must be valid for every member. source: S2 evidence: "valid for every member of the union"

Sources

• [S1] TypeScript Handbook: Narrowing. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/narrowing.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.
• [S2] Everyday Types. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/everyday-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.

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_z7nne9rj,
  author = {{iinventedlight}},
  title = {{TypeScript narrowing: typeof, instanceof, in, and type predicates}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-narrowing/},
  note = {Permanent ID iil:note:z7nne9rj. License: CC-BY-4.0.},
}

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