I'm baffled. I imagine I've tread on some bit of JavaScript esoterica but I don't know what it is.

The code is

processMessage(ws, json){
    let msg = JSON.parse(json);

    console.log(`Message(${msg.type}): ${msg.data}`);
    console.log("Available games: " + msg.data); // Normally isn't here, put it in to make sure I wasn't going crazy

    switch (msg.type){
        case "welcome":
            console.log("Welcome received. Requesting available games.");
            let msg = {
                "type":"game list",
                "data":null
            };

            ws.send(JSON.stringify(msg));
            break;
        case "restore":
            break;
        case "game list":
            console.log("Available games: " + msg.data);
            break;
        case "create console":
            break;
    }
}

It generates this output

Message(welcome): undefined                                                                 MainUI.js:94:21
Available games: undefined                                                                  MainUI.js:95:21
Welcome received. Requesting available games.                                               MainUI.js:98:29
Message(game list): The Office                                                              MainUI.js:94:21
Available games: The Office                                                                 MainUI.js:95:21
Uncaught ReferenceError: can't access lexical declaration 'msg' before initialization       MainUI.js:109:21
    processMessage http://localhost:8000/MainUI.js:109
    onmessage http://localhost:8000/MainUI.js:44
    loaded http://localhost:8000/MainUI.js:42
    onload http://localhost:8000/:1

I'm pretty sure that output has msg, or its properties, being accessed 1,2,3...4 times before the error occurs.

Aaaaaand, I just found the cause. Knew it was my fault but there's the esoterica.