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performance.memory.usedJSHeapSize

<html>

<body>
    <p id="memory" style="position: fixed; top:10px; left:10px; font-size: 1.5em;"></p>
    <script>
        setInterval(() => {
            document.getElementById("memory").innerHTML = performance.memory.usedJSHeapSize;
        }, 300);

    </script>
</body>

</html>