Plain JS Cascading selects of unknown depth

const selObject = {
  "-- Select Item --": {
    "Clothes": {
      "-- Select brands --": {
        "Levis": {
          "-- Select product --": {
            "Jeans": {
              "-- Select size --": [
                "38",
                "39",
                "40"
              ]
            },
            "Jackets": {
              "-- Select size --": [
                "41",
                "42",
                "43"
              ]
            }
          }
        }, // end Levis
        "Gucci": {
          "-- Select product --": {
            "Shoes": {
              "-- Select size --": [
                "45",
                "50",
                "55"
              ]
            },
            "Dresses": {
              "-- Select size --": [
                "8",
                "9",
                "10"
              ]
            }
          }
        } // end Gucci
      } // end brands  
    }, // End clothes
    "Gadgets": {
      "-- Select brands --": {
        "LG": {
          "-- Select product --": {
            "TVs": {
              "-- Select size --": [
                "38",
                "39",
                "40"
              ]
            },
            "Phones": {
              "-- Select size --": [
                "8",
                "9",
                "10"
              ]
            }
          }
        }, // end Levis
        "Apple": {
          "-- Select product --": {
            "Macbooks": {
              "-- Select size --": [
                "15",
                "17",
                "21"
              ]
            },
            "iPhones": {
              "-- Select size --": [
                "8",
                "9",
                "10"
              ]
            }
          }
        } // end Apple
      } // end brands
    } // end  Gadgets
  } // end items
} // end  


function fillDropdown(target, obj) {
  const sel = document.createElement("select");
  const sub = document.createElement("div");
  if (typeof obj !== "object") {
    sub.innerHTML = "<p>Thank you for your selection</p>";
    target.appendChild(sub);
    return;
  }
  target.appendChild(sel);
  target.appendChild(sub);
  const [title, value] = Object.entries(obj)[0];
  //add our title option
  const option1 = document.createElement("option");
  option1.innerText = title;
  sel.appendChild(option1);
  //now add the sub items
  const items = Object.entries(value);
  items.forEach(([k, v]) => {
    const option = document.createElement('option');
    option.innerText = k;
    sel.appendChild(option);
  });
  sel.addEventListener("change", () => {
    sub.innerHTML = "";
    if (sel.selectedIndex > 0) {
      const i = items[sel.selectedIndex - 1];    
      fillDropdown(sub, i[1]);
    }
  }); 
}


window.onload = function() {
  //createSel(selObject);
  fillDropdown(
    document.querySelector('#selContainer'),
    selObject
  );
}

select {
  display: block;
  width: 100%;
  padding: 10px;
}

<form name="myform" id="myForm">
  <div id="selContainer">
  </div>
</form>