问如何对多个选择字段求和，包括减去负数(减为零)

EN

<select class="select1 selectables" id="dropdown-1" name="dropdown1"> 
 <option disabled="disabled" selected="selected" value="">select</option> 
 <option value="10">Choice 1 (plus 10)</option>
 <option value="-5">Choice 2 (minus 5)</option>
 <option value="60">Choice 3 (plus 60)</option> 
</select>
 <br />
<select class="select2 selectables" id="dropdown-2" name="dropdown2">    
 <option disabled="disabled" selected="selected" value="">select</option>
 <option value="5">Choice A (plus 10)</option>
 <option value="15">Choice B (minus 10)</option>
 <option value="15">Choice C (plus 15)</option> 
 </select>
 <br />
 <select class="select3 selectables" id="dropdown-3" name="dropdown3">    
  <option disabled="disabled" selected="selected" value="">select</option>
  <option value="5">Choice ii (- 35)</option>
 <option value="15">Choice ii (plus 15)</option>
 <option value="12">Choice iii (plus 12)</option> 
 </select>

<script>
document.addEventListener("DOMContentLoaded", function(event) {
    var gg1 = new JustGage({
      id: "gg1",
      value: 0
      // removed elements, as this area isn't an issue
    });

document.getElementById('dropdown-1').addEventListener('change', (event) => {
      sum = 0;
      testvar = event.target.value;
      if (testvar > 0) {
        sum = (sum + testvar);
        alert(`up ${testvar}`);
      } else if (testvar < 1) {
        sum = (sum - parseInt(testvar, 10));
        alert(`down ${testvar}`);
      }
      gg1.refresh(sum);
      return false;
    });
</script>