flatDeepObject

const input = {
  a: '1',
  b: {
    c: '2',
    d: '3',
    e: {
      f: '4',
      g: {},
    },
  },
};

const flatDeepObject1 = (obj) =>
  Object.entries(obj).reduce(
    (acc, [key, val]) => (Object(val) === val ? { ...acc, ...flatDeepObject1(val) } : { ...acc, [key]: val }),
    {}
  );
const flatDeepObject2 = (obj) =>
  Object.entries(obj).reduce(
    (acc, [key, val]) => ({ ...acc, ...(Object(val) === val ? flatDeepObject2(val) : { [key]: val }) }),
    {}
  );

const getEntries = (obj) =>
  Object.entries(obj).flatMap(([key, val]) => (Object(val) === val ? getEntries(val) : [[key, val]]));
const flatDeepObject3 = (obj) => Object.fromEntries(getEntries(obj));

console.log(flatDeepObject1(input));
console.log(flatDeepObject2(input));
console.log(flatDeepObject3(input));