TypeScript - Index Types

It is easier to understand by looking at the code directly.

function pluck<T, K extends keyof T>(o: T, propertyNames: K[]): T[K][] {
  return propertyNames.map((n) => o[n]);
}

interface Car {
  manufacturer: string;
  model: string;
  year: number;
}

let taxi: Car = {
  manufacturer: "Toyota",
  model: "Camry",
  year: 2014,
};

let makeAndModel: string[] = pluck(taxi, ["manufacturer", "model"]);

let modelYear = pluck(taxi, ["model", "year"]);

An object called taxi has an interface called Car. And there is a function pluck, which checks if the object has items in an array as its property.

If you look at (o: T, propertyNames: K[]) this section right here, the first argument is an object, which is taxi in this case, and the second argument is an array of strings, which are properties of the object.

If you add another property like owner: string to the Car interface, the type keyof Car, or keyof T in this case, is automatically updated as well.

The second operator is T[K], which is the indexed access operator. Here T represents the object, and K represents keyof T, so the pluck function can be re-written like below;

function pluck<T, K extends keyof T>(o: T, propertyName: K): T[K] {
  return o[propertyName]; // o[propertyName] is of type T[K]
}