Alright, looks like Sinon.JS is what you are looking for. I've never used it before, but I did to answer your question.
You can replace the global function alert (which is actually window.alert) with a temporary function that will record the message that would have been displayed.
It's easy to do in javascript (window.alert = function(msg) { savedMsg = msg; }). So you could do that within your test.
The complexity comes only from cleaning up after you've run your test. That's where you need Sinon.JS which can integrate with QUnit. You'll need this integration script.
<html>
<head>
<script src="http://code.jquery.com/jquery-latest.js"></script>
<link rel="stylesheet" href="http://code.jquery.com/qunit/git/qunit.css" type="text/css" media="screen" />
<script type="text/javascript" src="http://code.jquery.com/qunit/git/qunit.js"></script>
<script type="text/javascript" src="sinon-1.1.1.js"></script>
<script type="text/javascript" src="sinon-qunit-0.8.0.js"></script>
<script>
function to_test() {
window.alert("I'm displaying an alert");
return 42;
}
$(document).ready(function(){
module("Module A");
test("first skip alert test ", function() {
var stub = this.stub(window, "alert", function(msg) { return false; } );
equals(42, to_test(), "to_test() should return 42" );
equals(1, stub.callCount, "to_test() should have invoked alert one time");
equals("I'm displaying an alert",stub.getCall(0).args[0], "to_test() should have displayed an alert" );
});
});
</script>
</head>
<body>
<h1 id="qunit-header">QUnit example</h1>
<h2 id="qunit-banner"></h2>
<div id="qunit-testrunner-toolbar"></div>
<h2 id="qunit-userAgent"></h2>
<ol id="qunit-tests"></ol>
<div id="qunit-fixture">test markup, will be hidden</div>
</body>
</html>
