JS: Change select option value when prompted?

Take a look at this code. I think this is what you are trying to do:

HTML

<select id="optionvals" onclick="change()">
  <option value="" selected>Option 1</option>
  <option value="30">Option 2</option>
  <option value="90">Option 3</option>
  <option value="custom">Option 4</option>
</select>

JS

function change() {
  var oItem = document.getElementById('optionvals');
  var value = oItem.options[oItem.selectedIndex].value;
  if(value == "custom") {
    alert("you've clicked b");
    value = prompt("Please enter a new number:", "60");
    oItem.options[oItem.selectedIndex].value = value;
    console.log(oItem.options[oItem.selectedIndex].value)
  }
}

This is done to prompt you only when the option selected value changes custom. Then, after selecting a custom value, it will rewrite that customoption element's value to the value you just entered in the prompt. After assigning the new value, I logged the value to show you that it is working.

Here is a fiddle : https://jsfiddle.net/ng7xvy05/