fune/testing/web-platform/tests/XMLHttpRequest/send-sync-blocks-async.htm

<!DOCTYPE html>
<html>
<head>
    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
    <title>XMLHttpRequest: sync requests should block events on pending async requests</title>
</head>

<body>
    <div id="log"></div>

    <script type="text/javascript">
        var test = async_test();

        var expect = ['sync 4', 'async 2', 'async 3', 'async 4']
        var actual = []

        test.step(function()
        {
            var xhr_async = new XMLHttpRequest()
            xhr_async.open('GET', 'resources/delay.py?ms=1000', true) // first launch an async request, completes in 1 second
            xhr_async.onreadystatechange = test.step_func(() => {
                actual.push('async ' + xhr_async.readyState)
                if(xhr_async.readyState === 4 && actual.indexOf('sync 4')>-1){
                    VerifyResult()
                }
            });
            xhr_async.send()

            test.step_timeout(() => {
                var xhr_sync = new XMLHttpRequest();
                xhr_sync.open('GET', 'resources/delay.py?ms=2000', false) // here's a sync request that will take 2 seconds to finish
                xhr_sync.onreadystatechange = test.step_func(() => {
                    actual.push('sync ' + xhr_sync.readyState)
                    if(xhr_sync.readyState === 4 && actual.indexOf('async 4')>-1){
                        VerifyResult()
                    }
                });
                xhr_sync.send()
            }, 10);

            function VerifyResult()
            {
                test.step(function()
                {
                    assert_array_equals(actual, expect);
                    test.done();
                });
            };
        });
    </script>
</body>
</html>