Tells you what's different in the incoming array when compared to exisiting array

arrayDifference.js

/**
 * Tells you what's different in the incoming array when compared to exisiting array
 * Either pass in array of objects or pass in array of strings. Don't pass mixed array.
 * 
 * @param incomingArray
 * @param existingArray
 * @param {'object'|'string'} compareType
 */
function difference(incomingArray, existingArray, compareType = 'object') {
  const convertToStringIfNecessary = v => compareType === 'object' ? JSON.stringify(v) : v;
  const convertToObjectIfNecessary = v => compareType === 'object' ? JSON.parse(v) : v;
  const incoming = JSON.parse(JSON.stringify(incomingArray)).map(convertToStringIfNecessary);
  const existing = JSON.parse(JSON.stringify(existingArray)).map(convertToStringIfNecessary);

  const result = [];
  for (let i = 0; i < incoming.length; i++) {
    if (existing.indexOf(incoming[i]) === -1) {
      result.push(incoming[i]);
    }
  }
  return result.map(convertToObjectIfNecessary);
}