[TypeScript] Query Properties with keyof and Lookup Types in TypeScript

The keyof operator produces a union type of all known, public property names of a given type. You can use it together with lookup types (aka indexed access types) to statically model dynamic property access in the type system.

Take away:

1. extends

2. keyof 

interface Todo {
    id: number;
    text: string;
    completed: boolean;
}

const todo: Todo = {
    id: 1,
    text: "Buy milk",
    completed: false
}

// K extends keyof T: K will be the unit types of 'id', 'text', 'completed'
// T[K] is the lookup tells the Typescript the correct return type
function prop<T, K extends keyof T>(obj: T, key: K): T[K] {
    return obj[key];
}

type TodoId = Todo['id'];
type TodoText = Todo['text'];
type TodoCompleted = Todo['completed'];

const id: TodoId = prop(todo, 'id'); // type number
const text: TodoText = prop(todo, 'text'); // type string
const completed: TodoCompleted = prop(todo, 'completed'); // type boolean