beep browser when a new DOM element appears

gistfile1.txt

[
    ["li pre.log-line", "Error", 440],
    ["li pre.log-line", "Warning", 523]
].forEach(([selector, targetText, frequency]) => { 
    new MutationObserver(mutations => 
        mutations.forEach(({ addedNodes }) => 
            addedNodes.forEach(node => {
                if (node.nodeType === 1 && 
                    (node.matches?.(selector) || node.querySelector?.(selector)) && 
                    node.textContent.includes(targetText)) {

                    console.log(`Matched: "${node.textContent.trim()}"`); 

                    const ctx = new (AudioContext || webkitAudioContext)();
                    const osc = ctx.createOscillator();
                    osc.type = "sine";
                    osc.frequency.setValueAtTime(frequency, ctx.currentTime);
                    osc.connect(ctx.destination);
                    osc.start();
                    setTimeout(() => osc.stop(), 200);
                }
            })
        )
    ).observe(document.body, { childList: true, subtree: true });
});