Why websocket sendMessage is required - websocket

Question

This is the code taken from the book "The Definitive Guide to HTML5 websocket"
div id="output"></div>
<script>
function setup() {
output = document.getElementById("output");
ws = new WebSocket("ws://localhost:7777");
ws.onopen = function(e) {
log("Connected");
sendMessage("Hello Websocket!");
}
ws.onclose = function(e){
log("Disconnected: " + e.reason);
}
ws.onerror = function(e){
log("Error ");
}
ws.onmessage = function(e) {
log("Message received: " + e.data);
ws.close();
}
}
function sendMessage(msg){
ws.send(msg);
log("Message Sent");
}
function log(s){
var p = document.createElement("p");
p.style.wordWrap = "break-word";
Could anybody please let me know for what reason this below event is required ??.
ws.send(msg);
I understand that the below will call the onMessage method on server side as shown below
public void onMessage(String data) {
}
But the actual purpose of onMessage on server side is to send data from backend to the javascript which will inturn call onmessage of client side javascript .
could anybody please help me understand this .

Answer 1

In the above code the ws.onopen, ws.onclose, ws.onmessage are all events associated with WebSocket object.
Whereas ws.send() is a method associated with WebSocket object. There is a huge difference between them.
The following event ensures that the Web Socket is connected to the server i.e you've opened your connection
ws.onopen = function(e) {
//Once you've opened your connection
//you can begin transmitting data to the server
//using the following method
ws.send("You\'re message");
}
So the main purpose of ws.send() method is to transmit data from the client to the server which is done by simply calling the WebSocket object's send() [in your case ws.send(msg)].
And also send data only takes place once a connection is established with the server by defining an onopen event handler.