setHiddenInputValues.js

var setHiddenInputValues = function (formId, urlParams) {
  var inputs = document.getElementById(formId).elements
  inputsArr = Array.apply(null, inputs)
  
  hiddenInputs = inputsArr.filter(function(i) {
    return i.type === 'hidden';
  })

  // set value of inputs to the url params passed in
  hiddenInputs.forEach(function(i) {
    key = i.name;
    val = urlParams[key];
    if (val) {
      i.value = val;
    }
  });
}