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chromium/third_party/blink/web_tests/fast/events/before-unload-returnValue.html 

<!DOCTYPE html>
<html>
<head>
<script src="../../resources/js-test.js"></script>
<script>
if (window.testRunner)
    testRunner.setShouldStayOnPageAfterHandlingBeforeUnload(true);

// JavaScript onbeforeunload dialogs require a user gesture.
if (window.eventSender) {
    eventSender.mouseMoveTo(5, 5);
    eventSender.mouseDown();
    eventSender.mouseUp();
}

window.onload = function() {
    window.location.href = "resources/does-not-exist.html";
    setTimeout(finishJSTest, 0);
}

var testEvent;
window.onbeforeunload = function(event) {
    testEvent = event;
    shouldBe("testEvent.__proto__", "BeforeUnloadEvent.prototype");
    shouldBeEqualToString("testEvent.returnValue", "");
    event.returnValue = "This is beforeunload from the top level frame.";
    shouldBeEqualToString("testEvent.returnValue", "This is beforeunload from the top level frame.");
}
</script>
</head>
<body>
<script>
description("Tests the returnValue attribute of the BeforeUnloadEvent.");
self.jsTestIsAsync = true;
</script>
</body>
</html>