Extracting Keys from Types

Mapped types don't just let you build new object types – they can also be used to extract keys. Say we have this object type:

type Soldier = {
  name: string;
  age: number;
  branch: "garrison" | "military police" | "survey corps";
};

Now imagine you want to get just the keys of the fields that are string-based – maybe for a filter, a dropdown, or feeding to an LLM that summarizes records. First, we create an object where each key either returns the key name, or never:

type StringKeys<T> = {
  [K in keyof T]: T[K] extends string ? K : never;
};

That gives you something like:

type Result = {
  name: "name";
  age: never;
  branch: "branch";
};

Now we index into that type using all of its keys:

type StringKeyUnion<T> = StringKeys<T>[keyof T];

We've made the object into a union of its values:

type Keys = StringKeyUnion<Soldier>;
// "name" | "branch"