ladybird/Tests/LibWeb/Text/input/Streams/TransformStream-start-callback.html

<script src="../include.js"></script>
<script>
    test(() => {
        const {readable} = new TransformStream({
            start(controller) {
                println("In start");
                controller.enqueue("Hello, world!");
            }
        });
        const reader = readable.getReader();
        reader.read().then(function processText({done, value}) {
            println(`Done: ${done}`);
            if (done)
                return;

            println(value);
            reader.read().then(processText);
        });
    });
</script>