Flatten object in Typescript

flatten.ts

interface NestedObject {
  [key: string]: NestedObject | string;
}

interface FlatObject {
  [key: string]: string;
}

const flatten = (input: NestedObject): FlatObject => {
  const result: FlatObject = {};
  const step = (obj: NestedObject | string, prefix: string) => {
    if (typeof obj !== 'object') {
      result[prefix] = obj;
      return;
    }
    for (const key in obj) {
      step(obj[key], prefix ? `${prefix}.${key}` : `${key}`);
    }
  };
  step(input, '');
  return result;
};

const stuff = {
  a1: {
    b1: {
      c1: "askjdlsakd",
      c2: "dsfjdskfj",
      c3: {
        d1: "djkfdjslfks"
      }
    }
  }
};

console.clear();
console.log(
  flatten(stuff),
);

// RESULT
// {
//   "a1.b1.c1": "askjdlsakd",
//   "a1.b1.c2": "dsfjdskfj",
//   "a1.b1.c3.d1": "djkfdjslfks"
// }