How do I keep multiple values from a checkbox from returning null in form submission in jquery/javascript?

I have a form that submits a survey. The form submits, however, when multiple checkboxes are checked and the form is submitted, it returns a null value in the database. Here is the Jquery used to submit the form:

    document.addEventListener("DOMContentLoaded", function(event) { 
  function getUrlQueryParams(url) {
  var queryString = url.split("?")[1];
  var keyValuePairs = queryString.split("&");
  var keyValue = [];
  var queryParams = {};
  keyValuePairs.forEach(function(pair) {
    keyValue = pair.split("=");
    queryParams[keyValue[0]] = decodeURIComponent(keyValue[1]).replace(/\+/g, " ");
  });
  return queryParams;
}

var paramMap = getUrlQueryParams(window.location.href);

// Get the PID and assign it
$(".Form[data-miniform-action='survey']").on('miniform::initialized', function(){
  $("input[name='q[2].text']").attr("value", paramMap["pid"]);
});
$(".Form[data-miniform-action='survey']").on('miniform::fieldsAdded', function(){
  $("input[name='q[2].text']").attr("value", paramMap["pid"]);
});

// Get the TID and assign it
$(".Form[data-miniform-action='survey']").on('miniform::initialized', function(){
  $("input[name='q[3].text']").attr("value", paramMap["tid"]);
});
$(".Form[data-miniform-action='survey']").on('miniform::fieldsAdded', function(){
  $("input[name='q[3].text']").attr("value", paramMap["tid"]);
});

// Get the EID and assign it
$(".Form[data-miniform-action='survey']").on('miniform::initialized', function(){
  $("input[name='q[4].text']").attr("value", paramMap["eid"]);
});
$(".Form[data-miniform-action='survey']").on('miniform::fieldsAdded', function(){
  $("input[name='q[4].text']").attr("value", paramMap["eid"]);
});

});

The only error I see when the submission is in the database is that the submissions seem to give the null values for multiple selections as a result of concatenating the id and the value (e.g. answerIds_257000, when it should be answerIds: 257000) Is there anything I can do to undo the concatenation for multiple Ids?