Conditional, Template Literal and Recursive Types

Key points

1. 1

   extends in a conditional type is an assignability check. infer X binds a type variable from the match and is usable only in the true branch.

2. 2

   A naked type parameter distributes over unions: each member is checked separately and the results are unioned. Wrap both sides in a tuple, [T] extends [U], to turn this off.

3. 3

   never is the empty union, so a distributive conditional over never returns never. any returns the union of both branches.

4. 4

   Several infer X candidates in covariant positions combine into a union; in contravariant positions (function parameters) they combine into an intersection. That is how UnionToIntersection works.

5. 5

   Template literal types multiply unions and can parse strings with infer; infer N extends number converts numeric strings into number literals.

6. 6

   Recursive conditional types in tail position run up to about 1,000 iterations (TS 4.5+); recursion inside spreads or other types hits the depth error far sooner. Use accumulators.

7. 7

   Inferring from an overloaded function uses its last signature, and a conditional return type on a generic function stays deferred, so returns need overloads or assertions.

Read the source

• TypeScript Handbook: Conditional Types
• TypeScript Handbook: Template Literal Types
• type-challenges