I'm trying to set up a Lightning component to listen for Platform Events and the above error occurs while CometD is handshaking.

What's interesting is I've tried the code in a Dev org and did not run into this issue (the error occurs in a sandbox).

This issue was mentioned in a comment here, but no solution was given.

The setup looks like this and is called after the CometD static resource has loaded (adapted from here)

doInit: function(component, event, helper) {
    var action = component.get("c.getSessionId");
    action.setCallback(this, function(response) {

        var sessionId = response.getReturnValue();
        var cometd = new window.org.cometd.CometD();
            url: window.location.protocol + '//' + window.location.hostname + '/cometd/41.0/',
            requestHeaders: { Authorization: 'OAuth ' + sessionId},
            appendMessageTypeToURL : false
        cometd.websocketEnabled = false;

        cometd.handshake($A.getCallback(function(status) {
            if (status.successful) {
                var eventName = component.get("v.channel");
                cometd.subscribe(eventName, $A.getCallback(function(message) {
                        var messageEvent = component.getEvent("onMessage");
                        messageEvent.setParam("payload", message.data.payload);
            } else {


And here is the error stack trace (redacted)

TypeError: Cannot add property context, object is not extensible
    at F.LongPollingTransport._self.xhrSend (https://xxx/cometd/cometd.js:564:25)
    at F.LongPollingTransport._self.transportSend (https://xxx/cometd/cometd.js:596:36)
    at F._transportSend (https://xxx/cometd/cometd.js:336:18)
    at F._queueSend (https://xxx/cometd/cometd.js:376:32)
    at F.RequestTransport._self.send (https://xxx/cometd/cometd.js:487:28)
    at _send (https://xxx/cometd/cometd.js:1657:24)
    at _handshake (https://xxx/cometd/cometd.js:1875:13)
    at CometD.handshake (https://xxx/cometd/cometd.js:2553:13)
    at Object.<anonymous> (https://xxx/apex/components/c/streaming.js:24:20)
    at n (https://xxx/auraFW/javascript/xxx/aura_prod.js:824:138)

Has anyone else run into this issue and found a solution?

  • 1
    This comment explains the error and possible workarounds.
    – sbordet
    Nov 3, 2017 at 9:10
  • How did I miss that? That fixed it, thanks!
    – kacrouse
    Nov 3, 2017 at 16:18

1 Answer 1


As sbordet said in the comment above, there was a solution (or at least workaround) given in the question I linked to, that I somehow missed.

Here is the workaround provided by Grekker in that thread:

The cometd library on GitHub since version 3.1.2 added a line of code that Salesforce doesn't like, my guess is because it changes XHR and LockerService is forcing XHR to not change. Here's the offending commit: github.com/cometd/cometd/commit/… To get around this use version 3.1.1, or reverse/comment out the additions in this commit.

Switching my CometD version to 3.1.1 fixed the issue.

  • this is still true and open issue with the latest versions of cometd. downgrading to 3.1.1 resolved my issue. I was getting handshake issue. Thank you
    – GirishP
    Sep 28, 2020 at 2:24

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