How to use SignalR to send data to a specific user?

If you are using Azure SignalR Service:

module.exports = async function (context, req) {    context.bindings.signalRMessages = [{        // message will only be sent to this user ID        "userId": "userId1",        "target": "newMessage",        "arguments": [ req.body ]    }];};

One userId could map to multiple client connections (e.g. devices), be aware of that.

If you need to send messages to multiple users or are hosting SignalR yourself:

Groups are the easiest way to send messages to a subset of users. If you want to send a message to a certain user, you can use the userId as the name of the group.

Deciding which user belongs to which group is a server-side feature, so you need to write some code.

module.exports = async function (context, req) {  context.bindings.signalRGroupActions = [{    "userId": req.query.userId,    "groupName": "myGroup",    "action": "add"  }];};

Using the Azure SignalR Service and the client-side code from the question I was able to get it to work. I used the following Azure Function to negotiate the connection:

module.exports = async function (context, req, connectionInfo) {  context.res.body = connectionInfo;  context.done();};

{  "disabled": false,  "bindings": [    {      "authLevel": "anonymous",      "type": "httpTrigger",      "direction": "in",      "name": "req"    },    {      "type": "http",      "direction": "out",      "name": "res"    },    {      "type": "signalRConnectionInfo",      "name": "connectionInfo",      "userId": "{userId}",             // <----- IMPORTANT PART!      "hubName": "chat",      "direction": "in"    }  ]}

As well as another function to send a message to a specific user:

module.exports = async function (context, req) {  const messageObject = req.body;  return {    "target": "newMessage",    "userId": messageObject.userId,    "arguments": [ messageObject.message]  };};

{  "disabled": false,  "bindings": [    {      "authLevel": "anonymous",      "type": "httpTrigger",      "direction": "in",      "name": "req",      "methods": [        "post"      ]    },    {      "type": "http",      "direction": "out",      "name": "res"    },    {      "type": "signalR",      "name": "$return",      "hubName": "chat",      "direction": "out"    }  ]}