How do conditional types work?

A conditional type has the form T extends U ? X : Y. It is a type-level if. The compiler checks assignability: if T is assignable to U, the result is X; otherwise it is Y. Conditional types are the foundation of utility types such as Exclude and Extract.

type IsString<T> = T extends string ? true : false;

type A = IsString<"x">;
type B = IsString<number>;

type ToArray<T> = T extends unknown ? T[] : never;
type Result = ToArray<string | number>;

Distribution and its escape hatch

• A naked type parameter distributes over each member of a union
• Wrapping both sides in tuples stops distribution
• Distributive conditionals are how Exclude and Extract filter unions
• Conditional types are lazy until the type arguments are known
• Nested conditionals can model more than two branches

type IsUnion<T> =
  [T] extends [infer U]
    ? ([U] extends [T] ? false : true)
    : false;