deepMerge.js

function deepMerge(...objects) {
  let dest = objects[0];
  let target = objects[1] || {};

  for(let targetKey of Object.keys(target)) {
    let targetValue = target[targetKey];

    if (Array.isArray(targetValue)) {
      dest[targetKey] = dest[targetKey] || [];
      dest[targetKey].push(...targetValue);
    }
    else if (typeof targetValue === 'object' && targetValue) {
      dest[targetKey] = merge(dest[targetKey] || {}, targetValue);
    }
    else {
      dest[targetKey] = targetValue;
    }
  }

  return objects.length > 2 ?
    merge(dest, ...objects.slice(2)) :
    dest;
}