code: function _deepFlatten (nestedObject, flattenedObject) {
    for (const prop in nestedObject) {
      if (nestedObject.hasOwnProperty(prop)) {
        const value = nestedObject[prop]
        if (typeof value === 'object' && value !== null) {
          _deepFlatten(value, flattenedObject)
        } else {
          flattenedObject[prop] = value
        }
      }
    }
  }
docs: helper function used by deepFlatten