socket.io-client vs socket.io

Question

What's the difference between socket.io-client and socket.io ?

I also find a bit confusing that socket.io-client also has a section for "server-side usage"

Answer 1

socket-io.client is the code for the client-side implementation of socket.io. That code may be used either by a browser client or by a server process that is initiating a socket.io connection to some other server (thus playing the client-side role in a socket.io connection).

A server that is not initiating socket.io connections to other servers would not use this code. This has been made a little more confusing that it probably should be because when using socket.io, it appears that both client and server are using the same socket.io.js file (because they both refer to a file with the same name), but is not actually the case. The server is using a different file than the client.

From the Github page for socket-io.client :

A standalone build of socket.io-client is exposed automatically by the socket.io server as /socket.io/socket.io.js. Alternatively you can serve the file socket.io.js found at the root of this repository.

Keep in mind that there are unique features that belong to client and server so it should not be a surprise that they use some different code. Though they share code for parsing the protocol and things like that, the server has the ability to run a server or hook into an existing web server and it has methods like .join() and .leave() and data structures that keep track of all the connected sockets and is expected to live in the node.js environment. The client has the ability to initiate a connection (send the right http request), do polling if webSockets are not supported, build on a native webSocket implementation if present, etc....