Function keysOf

Returns the keys of an object of type T. This is like Object.keys except the return type captures the known keys of T.

Note that it is the responsibility of the caller to use this wisely -- there are cases where the runtime set of keys returned may be broader than the type checked set at compile time, so there's potential for this to be abused in ways that are not inherently type safe. For example, given base class Animal, subclass Fish, and const animal: Animal = new Fish(); then keysOf(animal) will not type-check the entire set of keys of the object animal since it is actually an instance of type Fish, which has an extended property set.

In general, it should be both convenient and type-safe to use this when enumerating the keys of simple data objects with known properties.

interface Point { x: number; y: number; }
const point: Point = { x: 1, y: 2 };
const keys = keysOf(point);
// type of keys -> ('a' | 'b')[]
for (const key of keys) {
  console.log(key, point[key]);
}
// x 1
// y 2
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