I have 4 selects and I'm trying to get the device and the phone or watch and model of the device and finally the problem with the device, so every is working untill i choose the device and then the phone but when i try to choose the model of the phone there is no values, I couldn't figure it out, This is my code.

var subjectObject = {
  Phone: {
    Samsung: {
      A50: [
        "Screen",
        "Screen High Copy",
        "Battery",
        "Charging Port",
        "Speaker",
        "Front Camera",
        "Back Camera",
        "Home Button",
        "Loud Speaker",
        "IC Brightness",
        "IC Charging",
        "IC Display",
        "IC Touch",
        "IC WI-FI",
        "IC Baseband",
        "Short Circuit",
      ],
      A70: [
        "Screen",
        "Screen High Copy",
        "Battery",
        "Charging Port",
        "Speaker",
        "Front Camera",
        "Back Camera",
        "Home Button",
        "Loud Speaker",
        "IC Brightness",
        "IC Charging",
        "IC Display",
        "IC Touch",
        "IC WI-FI",
        "IC Baseband",
        "Short Circuit",
      ],
    },
    
  
  Watch: {
    Apple: {
      SmartWatch1: ["Broken Screen"],
    },
  },
};
window.onload = function () {
  var device = document.getElementById("device");
  var phone = document.getElementById("phone");
  var model = document.getElementById("model");
  var problem = document.getElementById("problem");
  for (var x in subjectObject) {
    device.options[device.options.length] = new Option(x, x);
  }
  device.onchange = function () {
    phone.length = 1;
    model.length = 1;
    problem.length = 1;
    //display correct values
    for (var y in subjectObject[this.value]) {
      phone.options[phone.options.length] = new Option(y, y);
    }
  };
  phone.onchange = function () {
    model.length = 1;
    //display correct values
    var z = subjectObject[phone.value][this.value];
    for (var i = 0; i < z.length; i++) {
      model.options[model.options.length] = new Option(z[i], z[i]);
    }
  };
  model.onchange = function () {
    problem.length = 1;
    //display correct values
    var z = subjectObject[model.value][this.value];
    for (var i = 0; i < z.length; i++) {
      problem.options[problem.options.length] = new Option(z[i], z[i]);
    }
  };
};