arr1 = [{a: 1, b: 2}, {a: 15, b: 4}, {a: 12, b: 7}];
arr2 = [{a: 15, b: 4}, {a: 1, b: 2}, {a: 12, b: 7}];

arr1 = [{a: 1, b: 2}, {a: 15, b: 4}, {a: 12, b: 7}];
arr2 = [{a: 15, b: 4}, {a: 1, b: 3}, {a: 12, b: 7}];

arr.sort((obj1, obj2) => {
    return *???*
});

arr.sort((obj1, obj2) => {
  var result = obj1.a - obj.a;
  if (result == 0)
    result = obj1.b - obj.b;
  return result;
});