[SuperZen]

<script>
  const users = {
    '727219': {
      id: 100
    },
    '727220': {
      id: 101
    }
  }

  function userById(id, users) {
    for (user in users) {
      if (users[user].id === id) return {
        id,
        user
      }
    }
  }

  console.log(userById(100, users))
</script>