const a = {
  "2100": 30,
  "2101": 31,
  "2102": 32,
  "2103": 33,
};

const b = {
  "2100": 31,
  "2101": 32,
  "2102": 33,
  "2103": 34,
  "2104": 35,
  "2105": 36,
  "2106": 37
};

for (let el in b) {
  if (!a[el]) {
    a[el] = a[el - 1];
  }

  a[el] = b[el];
  console.dir({ element: el, value: a });
}