mergeDeepWithoutLoop.js

const isObject = value => typeof value === 'object' && value !== null;

const mergeDeepWithoutLoop = (target, source) => {
  source = new Proxy(source, {
    get: function(source, property) {
      const value = source[property];

      if (isObject(value)) {
        return mergeDeepWithoutLoop(target[property], value);
      }

      return value;
    }
  });

  return { ...target, ...source };
};

const target = { a: { b: { c: 1 }, e: { a: 1 } }, f: 4 };
const source = { a: { b: { d: 2 }, e: { b: 2 } }, g: 4 };

console.log(mergeDeepWithoutLoop(target, source)) // {"a":{"b":{"c":1,"d":2},"e":{"a":1,"b":2}},"f":4,"g":4}