Deep merge two objects in javascript

mergeObjects.js

function mergeObjects(target, source, mergeArrayWith) {
  if (!mergeArrayWith) {
    mergeArrayWith = (targetArray, sourceArray) => [...targetArray, ...sourceArray];
  }

  Object.keys(source).forEach(key => {
    const existingValue = target[key];
    const valueToBeMerged = source[key];

    if (Array.isArray(existingValue)) {
      if (!Array.isArray(valueToBeMerged)) {
        throw Error('type of key ' + key + ' must match in both objects');
      }
      target[key] = mergeArrayWith(existingValue, valueToBeMerged);
    } else if (typeof existingValue === 'object') {
      if (typeof valueToBeMerged !== 'object' || Array.isArray(valueToBeMerged)) {
        throw Error('type of key ' + key + ' must match in both objects');
      }
      mergeObjects(existingValue, valueToBeMerged);
    } else {
      target[key] = valueToBeMerged;
    }
  });
  return target;
};