Virtual generated columns

<div class="demo"><div class="head"><b>GENERATED ALWAYS AS</b><span>virtual</span></div><div class="stage calc"><div class="cell">price<br><strong>12</strong></div><span>×</span><div class="cell">qty<br><strong id="qty">2</strong></div><span>=</span><div class="cell total">total<br><strong id="total">24</strong></div></div><div class="foot"><span id="calcstatus">read computes 12 × 2</span><span>not stored</span></div></div>

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let n=0;function tick(){let qty=n?3:2;document.getElementById('qty').textContent=qty;document.getElementById('total').textContent=12*qty;document.getElementById('calcstatus').textContent='read computes 12 × '+qty;document.querySelector('.total').classList.toggle('on',!!n);n=1-n}tick();const t=setInterval(tick,1100);document.querySelector('.demo').onclick=()=>{clearInterval(t);tick()}