var helperList = ['a', 'b', 'c', 'd', 'e'];
var documentedList = ['a', 'c'];

var undocumentedList = [];
helperList.forEach(function(helper) {
  if(_.contains(documentedList, helper) === false)  {
    undocumentedList.push(helper);
  }
});

console.log(undocumentedList);