Generic Constraints

Generic Constraints

Sometimes you need a generic that accepts any type — but only types with certain properties. Use extends to constrain the type parameter:

interface HasLength {
    length: number;
}

function longest<T extends HasLength>(a: T, b: T): T {
    return a.length >= b.length ? a : b;
}

console.log(longest("hello", "hi"));         // hello
console.log(longest([1, 2, 3], [1, 2]));    // [1, 2, 3]

T extends HasLength means: T can be any type that has a length: number property. Both string and Array qualify.

keyof Constraint

keyof T is the union of a type's property names. It lets you write functions that safely access an object's properties:

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

const user = { name: "Alice", age: 30 };
console.log(getProperty(user, "name"));  // Alice
console.log(getProperty(user, "age"));   // 30
// getProperty(user, "missing");         // Error: not a key of user

Your Task

Write function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] and use it to access properties of an object.