What are Conditional Types in TypeScript?

// Syntax: T extends U ? X : Y
type IsString<T> = T extends string ? true : false;

type A = IsString<string>;  // true
type B = IsString<number>;  // false

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

type NumArr = ElementType<number[]>;  // number
type StrArr = ElementType<string[]>;  // string

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

type Fn = () => string;
type Result = ReturnType<Fn>;  // string

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

type StrOrNum = ToArray<string | number>;
// string[] | number[]

// Built-in utility types using conditional types
type Exclude<T, U> = T extends U ? never : T;
type Extract<T, U> = T extends U ? T : never;

type T1 = Exclude<"a" | "b" | "c", "a">;  // "b" | "c"
type T2 = Extract<"a" | "b" | "c", "a" | "b">;  // "a" | "b"