InvalidStateError on websocket object

Question

I'm just starting learning websockets, andI get a strange error:

<script type="text/javascript">
    window.onload = function(){
        var service = new WebSocket("ws://localhost:8080/websocket/test"); 
        service.onmessage = function(event){
            alert("message"); 
        } 
        service.onopen = function(){
            service.send("hello!");
        } 
        service.onclose = function(){
            alert("closed"); 
        } 
        service.onerror = function(){
            alert("error"); 
        }

        service.send("test");
        service.close();
    }

</script>

on the line:

            service.send("test");

I get:

InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

Am I missing something important?

Answer 1

The WebSocket.onopen property is an EventHandler that is called when the WebSocket connection's readyState changes to OPEN; this indicates that the connection is ready to send and receive data.

Once you've opened your connection, you can begin transmitting data to the server.

window.onload = function() {
  var service = new WebSocket("wss://echo.websocket.org");
  service.onmessage = function(event) {
    alert("onmessage event: "+event.data);
  }
  service.onopen = function() {
    service.send("test"); //Will work here!
    //^^^^^^^^^^^^^^^^^
    service.send("hello!");
  }
  service.onclose = function() {
    alert("closed");
  }
  service.onerror = function() {
    alert("error");
  }

  //Can't close while a connection is still being established.
  //service.close();
}

Answer 2

If you use Websocket().send() outside your service.onopen(), you can still check the readyStatebefore sending message, like this:

    websocket.sendmessage = async function(msg) {
       while (this.readyState === 0) {
            await sleep(200);
       }
       this.send(msg);
    };