When non of the options are selected for the checkboxes, how to make the value none?

I've my code below and I'm using an array here. I'm trying to print "None" value when none of the options are selected, but instead when I submit the form I have a few checkboxes selected; it still says "None". Without the else if condition when I choose to not select one of the option, then I get nothing for the result.

I'm not supposed to create a "None" option for the input function.

function calculatePrice() {
  var array = [];
  var extras = document.calc.extras;

  for (count = 0; count < extras.length; count++) {
    if (extras[count].checked == true) {
      array.push(extras[count].value);
    } else if (!extras[count].checked) {
      array = ["None"];
    }
  }
  alert("(Including extras: " + array + ")");
}

<form name="calc">
  <table>
    <tr>
      <td colspan="40" id="col">
        <label for="extras">EXTRAS</label>
        <div class="left">
          <input type="checkbox" name="extras" id="ac" value="A/C">A/C (+$10)
          <input type="checkbox" name="extras" id="work" value="Working Brakes">Working Brakes (+$100)<br>
          <input type="checkbox" name="extras" id="cruise" value="Cruise Control">Cruise control (+$20)
          <input type="checkbox" name="extras" id="seat" value="Baby Seat">Baby Seat (+$30)<br>
        </div>
      </td>
    </tr>
    <tr>
      <td colspan="40" id="col">
        <input type="submit" onClick="calculatePrice();" value="Estimate Cost">
      </td>
    </tr>
  </table>
</form>