TypeScript utility types

utility.ts

export type Primitive<T> = T extends string
  ? string
  : T extends number
  ? number
  : T extends boolean
  ? boolean
  : T extends Record<keyof any, unknown>
  ? Lookalike<T>
  : never;

export type Lookalike<T> = {
  [K in keyof T]: Primitive<T[K]>;
};
  
// eg.
const config = {
  dbName: 'foo',
  collectionName: 'bar',
} as const; // note this is a const literal

// illegal
config.dbName = 'changed';

// illegal
const fake: typeof config = {
  dbName: 'baz',
  collectionName: 'bat',
};
  
// permitted
const testConfig: Lookalike<typeof config> = {
  dbName: 'bob',
  collectionName: 'belcher',
};