socket.io

• https://cdn.bootcdn.net/ajax/libs/socket.io/2.1.1/socket.io.dev.
• https://cdn.bootcdn.net/ajax/libs/socket.io/2.1.1/socket.io.
• https://cdn.bootcdn.net/ajax/libs/socket.io/2.1.1/socket.io.slim.dev.
• https://cdn.bootcdn.net/ajax/libs/socket.io/2.1.1/socket.io.slim.