GenericsLesson 4.2

Generic constraints in TypeScript using extends

extends constraint, constraining to interface, keyof constraint, multiple constraints, why unconstrained generics fail

Why constraints are needed

An unconstrained T cannot access any properties — TypeScript does not know what T looks like:

function getName(obj: T): string {
  return obj.name; // Error: Property 'name' does not exist on type 'T'
}

Add a constraint with extends:

function getName(obj: T): string {
  return obj.name; // safe
}

getName({ name: "Alice", age: 30 }); // works
getName({ id: 1 });                  // Error: missing name

keyof constraint

function getProperty(obj: T, key: K): T[K] {
  return obj[key];
}

const user = { name: "Alice", age: 30 };
getProperty(user, "name");  // string
getProperty(user, "age");   // number
getProperty(user, "email"); // Error: not a key of user

The keyof T produces a union of all keys of T as string literals. T[K] is an indexed access type — the type of the value at key K. Together they make getProperty fully type-safe.