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Short answer

type IsString<T> = T extends string ? "yes" : "no";

If the left side is assignable to the right, you get the first branch; otherwise the second. [S1][S2]

Two key tricks

• Conditional types provide us with a way to infer from types we compare against in the true branch using the infer keyword. (S1) U : T;` [S1]

Conditional types are one of several type operators (with keyof, indexed access, mapped and template literal types) for building types from other types. [S2]

Claims

• id: C1 claim: If the left type is assignable, the true branch type is chosen. source: S1 evidence: "When the type on the left of the extends is assignable to the one on"
• id: C2 claim: The infer keyword infers types in the true branch. source: S1 evidence: "infer from types we compare against in the true branch using the infer keyword"
• id: C3 claim: Conditional types distribute over unions given to a generic. source: S1 evidence: "they become distributive when given a union type"
• id: C4 claim: Conditional types act like if statements in the type system. source: S2 evidence: "Types which act like if statements in the type system"

Sources

• [S1] TypeScript: Documentation - Conditional Types. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/conditional-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
• [S2] TypeScript: Documentation - Creating Types from Types. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/types-from-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_qzanjw5n,
  author = {{iinventedlight}},
  title = {{TypeScript conditional types and infer}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-conditional-types/},
  note = {Permanent ID iil:note:qzanjw5n. License: CC-BY-4.0.},
}

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