Lesson 21 — Conditional types

Types with an if

A conditional type chooses between two types: T extends U ? X : Y reads "if T fits U, then X, else Y". Inside the condition, infer captures a part of the type into a new name — T extends (infer U)[] grabs the item type of an array.

type IsText<T> = T extends string ? "yes" : "no";
type A = IsText<"hi">;  // "yes"
type B = IsText<42>;    // "no"

Why it matters: library types (like the result type of a Promise) are built this way.