block9.ts

function pluck<A, P extends (keyof A)>(array: Array<A>, prop: P): Array<A[P]> {
    return array.map(el => el[prop]);
}

pluck([user1, user2], 'firstName') //will be an Array<string>
pluck([user1, user2], 'age') //will be an Array<number>
pluck([user1, user2], 'name') //will not compile Argument of type '"name"' is not assignable to parameter of type '"firstName" | "lastName" | "age"'