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Editorial Team
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Asked: 2026-05-27T16:08:49+00:00

I have a script which can be run either directly or, when available in

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I have a script which can be run either directly or, when available in the browser, as a Web Worker. I’d like to run a portion of this script only when run as a worker; so my question is, how can a script identify itself as being run this way?

I can’t see anything in the spec that would allow this to happen; am I missing something obvious?

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1 Answer

  1. Editorial Team

    In the following :

    <html>
<head>
<title>Worker</title>
</head>
<body>
</body>
<script >
  var w = new Worker ('worker.js');
  w.onmessage = function (e) {
    document.body.innerHTML += '<br>' + 'WORKER : ' + e.data;
  };
</script>
<script src='worker.js'></script>
</html>

    worker.js is invoked both as a script and as a worker.

    worker.js contains :

      var msg = 'postMessage is ' + postMessage.toString () + 
          ', self.constructor is ' + self.constructor;
  try {
    postMessage (msg);
  } catch (e) {
    document.body.innerHTML += '<br>SCRIPT : ' + msg;
  }

    In the worker environment, the postMessage succeeds, in the script environment it fails because either it is undefined or, in a browser, it requires a second argument.

    Output is :

    chrome :

    SCRIPT : postMessage is function () { [native code] }, self.constructor is function DOMWindow() { [native code] } 
WORKER : postMessage is function postMessage() { [native code] }, self.constructor is function DedicatedWorkerContext() { [native code] }

    firefox :

    SCRIPT : postMessage is function postMessage() { [native code] }, self.constructor is [object Window]
WORKER : postMessage is function postMessage() { [native code] }, self.constructor is function DedicatedWorkerGlobalScope() { [native code] }

    opera:

    WORKER : postMessage is function postMessage() { [native code] }, self.constructor is function Object() { [native code] }
SCRIPT : postMessage is function postMessage() { [native code] }, self.constructor is function Object() { [native code] }

    All under Ubuntu.

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