function flatten(source, key) {
  let it, index = 0;
  const flattened = {};
  const delim = key !== undefined ? '.' : '';
  key = key || '';

  if (typeof source === 'string') {
    flattened[key] = source;
    return flattened;
  }

  for (const prop in source) {
    it = source[prop];
    // STEP 2 - determine if the value property is an object, array, or value
    if (Array.isArray(it)) {
      // STEP 2.1 - it's an array, iterate over the array and get key for each obj
      index = 0;
      for (const obj of it) {
        key = delim + prop + '.' + index;
        Object.assign(flattened, flatten(obj, key));
        index++;
      }
    } else if (typeof it === 'object' && it !== null) {
      // STEP 2.2 - it's an object, traverse
      Object.assign(flattened, flatten(it, key + delim + prop));
    } else {
      // STEP 2.3 - The leaf - set the value
      flattened[key + delim + prop] = it;
    }
  }
  return flattened;
}