Will websocket messages always arrive entirely, at once? - http

Question

This websockets tutorial https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API/Writing_WebSocket_client_applications has this example:
exampleSocket.onmessage = function(event) {
var f = document.getElementById("chatbox").contentDocument;
var text = "";
var msg = JSON.parse(event.data);
var time = new Date(msg.date);
var timeStr = time.toLocaleTimeString();
switch(msg.type) {
case "id":
clientID = msg.id;
setUsername();
break;
case "username":
text = "<b>User <em>" + msg.name + "</em> signed in at " + timeStr + "</b><br>";
break;
case "message":
text = "(" + timeStr + ") <b>" + msg.name + "</b>: " + msg.text + "<br>";
break;
case "rejectusername":
text = "<b>Your username has been set to <em>" + msg.name + "</em> because the name you chose is in use.</b><br>"
break;
case "userlist":
var ul = "";
for (i=0; i < msg.users.length; i++) {
ul += msg.users[i] + "<br>";
}
document.getElementById("userlistbox").innerHTML = ul;
break;
}
if (text.length) {
f.write(text);
document.getElementById("chatbox").contentWindow.scrollByPages(1);
}
};
it parses the received message from the websockets server. However, can I trust that a json, sent by the websocket server, will always arrive at once for the client? What if it arrives partially? Parsing it would break it.
Can I make this assumption even for very large json messages?
I'm asking because on a TCP stream, this wouldn't be possible. The message could arrive partially.
If there is no way to receive the message entirely at once, how could I know when to parse a JSON?

Answer 1

Yes, each message is received as a whole. From the RFC 6455:
1.2. Protocol Overview
After a successful handshake, clients and servers transfer data back and forth in conceptual units referred to in this specification as "messages". On the wire, a message is composed of one or more frames. The WebSocket message does not necessarily correspond to a particular network layer framing, as a fragmented message may be coalesced or split by an intermediary.
6.2. Receiving Data
If the frame comprises an unfragmented message (Section 5.4), it is said that A WebSocket Message Has Been Received with type /type/ and data /data/. If the frame is part of a fragmented message, the "Application data" of the subsequent data frames is concatenated to form the /data/. When the last fragment is received as indicated by the FIN bit (frame-fin), it is said that A WebSocket Message Has Been Received with data /data/ (comprised of the concatenation of the "Application data" of the fragments) and type /type/ (noted from the first frame of the fragmented message). Subsequent data frames MUST be interpreted as belonging to a new WebSocket message.
https://www.rfc-editor.org/rfc/rfc6455
The confusion might come from the term 'socket' which is a low-level raw OS channel that yields data in chunks. However, WebSocket is a higher level protocol.