for-in delete

var object = {
  __proto__: {
  },

  a: 1,
  b: 2,
  c: 3
}

for (var x in object) {
  print(x);

  if (x == "b") {
    delete object.c;
  }
}

print(" === ");

var object2 = {
  __proto__: {
    c: 3
  },

  a: 1,
  b: 2,
}

for (var x in object2) {
  print(x);

  if (x == "b") {
    delete object2.__proto__.c;
  }
}

print(" === ");

var object3 = {
  __proto__: {
  },

  a: 1,
  b: 2,
  c: 3
}

for (var x in object3) {
  print(x);

  if (x == "b") {
    object3.__proto__.c = 30;
    delete object3.c;
  }
}