Generic Constraints

Constraining Generics with extends

interface Lengthwise { length: number; }

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

logLength([1, 2, 3]); // OK – arrays have length
logLength("hello");   // OK – string has length
// logLength(123);     // ❌ number doesn't have length

Using keyof to constrain property names

function getProp<T, K extends keyof T>(obj: T, key: K) {
  return obj[key];
}

const person = { name: "Bob", age: 30 };
const name = getProp(person, "name"); // inferred as string

Conditional Types with Generics

type NonNullable<T> = T extends null | undefined ? never : T;
type NN = NonNullable<string | undefined>; // string