Generic Types

Generics allow you to write reusable code that works with multiple types.

Generic Function

function identity<T>(arg: T): T {
  return arg;
}

const num = identity<number>(42);
const str = identity<string>("hello");
const auto = identity(true);  // Type inferred

Generic Interface

interface Box<T> {
  value: T;
}

const numberBox: Box<number> = { value: 123 };
const stringBox: Box<string> = { value: "hello" };

Generic Constraints

interface HasLength {
  length: number;
}

function logLength<T extends HasLength>(arg: T): void {
  console.log(arg.length);
}

logLength("hello");      // OK
logLength([1, 2, 3]);    // OK
// logLength(123);       // Error: number has no length

Multiple Type Parameters

function pair<T, U>(first: T, second: U): [T, U] {
  return [first, second];
}

const result = pair<string, number>("age", 30);