ObjectLeafKeys.test.ts

import type {ObjectLeafKeys} from '../ObjectLeafKeys';

type Expect<T, U> = T[] extends U[] ? (U[] extends T[] ? true : false) : false;

// jest always returns true, but type-checking will fail if T is false
const assert = <T extends true>() => {
  return true as T;
};

describe('ObjectLeafKeys', () => {
  it('infers the type of an object keys', () => {
    const test = {
      one: {
        two: {three: 'string'},
        four: 'string',
      },
      five: 'string',
    };

    type Result = ObjectLeafKeys<typeof test>;
    type Expected = 'five' | 'one.four' | 'one.two.three';

    expect(assert<Expect<Result, Expected>>()).toBe(true);
  });
});

ObjectLeafKeys.ts

/**
 * ObjectLeafKeys: Recursively enumerate all the leaf keys of an object
 * @example
 * type Test = {
 *   a: {
 *     b: { c: string; };
 *     d: string;
 *   };
 *   e: string;
 * };
 *
 * type Result = ObjectLeafKeys<Test>;
 * // Result = "a.b.c" | "a.d" | "e"
 */
export type ObjectLeafKeys<ObjectType extends object> = {
  [Key in keyof ObjectType & (string | number)]: ObjectType[Key] extends object
    ? `${Key}.${ObjectLeafKeys<ObjectType[Key]>}`
    : `${Key}`;
}[keyof ObjectType & (string | number)];