What is conditional generic typing — how do you write T extends U ? X : Y?

Conditional types select a type based on a condition, similar to a ternary operator for types.

// Basic syntax: T extends U ? TypeIfTrue : TypeIfFalse
type IsString<T> = T extends string ? 'yes' : 'no';

type A = IsString<string>;  // 'yes'
type B = IsString<number>;  // 'no'

Distribution over union types — when T is a naked type parameter, conditional types distribute:

type ToArray<T> = T extends unknown ? T[] : never;

type StringOrNumberArray = ToArray<string | number>;
// Distributes: (string extends unknown ? string[] : never) | (number extends unknown ? number[] : never)
// = string[] | number[]

// Prevent distribution with []
type ToArrayNonDist<T> = [T] extends [unknown] ? T[] : never;
type NonDist = ToArrayNonDist<string | number>; // (string | number)[]

infer keyword — extract a type within a conditional:

// Extract the return type of a function
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

type NumReturn = ReturnType<() => number>; // number
type VoidReturn = ReturnType<() => void>;  // void

// Extract element type from an array
type ElementType<T> = T extends (infer E)[] ? E : never;

type Elem = ElementType<string[]>; // string