KeyOf Operator

Question 1

Q: What is the keyof type operator in TypeScript and what does it return?

Answer 1

A: The keyof operator takes an object type and produces a union type of its keys (either string or numeric literals). It returns all property names as a union.

type Point = { x: number; y: number };
type PointKeys = keyof Point; // type PointKeys = "x" | "y"

// Usage example
function getPropertyValue<T, K extends keyof T>(obj: T, key: K): T[K] {
    return obj[key];
}

Question 2

Q: What are some practical use cases for the keyof operator?

Answer 2

A: The keyof operator is commonly used in generic functions to ensure type safety when accessing object properties and in mapped types.

// Type-safe object property access
function pluck<T, K extends keyof T>(obj: T, keys: K[]): T[K][] {
    return keys.map(key => obj[key]);
}

interface Person {
    name: string;
    age: number;
}

const person: Person = { name: "John", age: 25 };
const values = pluck(person, ["name"]); // Type safe: ["name"] must be keys of Person