How to get ID from checked Checkbox in Javascript (google script-app)

Question

How do I get the ID from a checked checkbox? I need to get the selected checkboxes values added into a Google Spreadsheets Cell after pressing submit.

function sendForm() {
  var checkedid = I WOULD NEED THE ID HERE ? ;
  var data = document.forms['form'].elements[checkedid].value;
  google.script.run.withSuccessHandler(ready).processForm(data);
  document.getElementById("all").innerHTML = "Processing..";
}

function ready() {
  document.getElementById("all").innerHTML = "Selected Gear Added!";
  google.script.run.showDialog();
}

<div id="all">
  <form id="form">
    <input class="checkboxclass" type="checkbox" id="id_1" value="Sony FS5 - SET 1">Sony FS5 - SET 1<br>
    <input class="checkboxclass" type="checkbox" id="id_2" value="Sony FS5 - SET 2">Sony FS5 - SET 1<br>
    <input class="checkboxclass" type="checkbox" id="id_3" value="Tripod">Tripod<br>
    <input class="checkboxclass" type="checkbox" id="id_4" value="Monopod">Monopod<br>
    <input class="checkboxclass" type="checkbox" id="id_5" value="Wireless pack - SET 1">Wireless pack - SET 1<br>
    <input class="checkboxclass" type="checkbox" id="id_6" value="Wireless pack - SET 2">Wireless pack - SET 2<br>
    <input class="checkboxclass" type="checkbox" id="id_7" value="Wireless pack - SONY">Wireless pack - SONY<br>
  </form><br><br>
  <button onclick="javascript:sendForm();">Submit</button>
</div>

Thanks for your help.