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Basic Fixed-Height List ​

Problem ​

You need to render thousands of list items efficiently. Every row is the same height, so the offset table can be computed once without measuring individual elements.

Solution ​

The simplest case: every row is the same height. The offset table is built once and never needs rebuilding.

ts
import { createVirtualizer } from '@vielzeug/virtualit';

const data = Array.from({ length: 50_000 }, (_, i) => ({ id: i, label: `Item ${i}` }));

const scrollEl = document.getElementById('scroll')!;
const listEl = document.getElementById('list')!;

const virt = createVirtualizer(scrollEl, {
  count: data.length,
  estimateSize: 40,
  onChange: (virtualItems, totalSize) => {
    listEl.style.height = `${totalSize}px`;
    listEl.innerHTML = '';

    for (const item of virtualItems) {
      const el = document.createElement('div');
      el.className = 'row';
      el.style.cssText = `position:absolute;top:${item.start}px;left:0;right:0;height:40px;line-height:40px;padding:0 12px;`;
      el.textContent = data[item.index].label;
      listEl.appendChild(el);
    }
  },
});
html
<div id="scroll" style="height:400px;overflow:auto;position:relative;">
  <div id="list" style="position:relative;"></div>
</div>

Pitfalls ​

  • Calling createVirtualizer before the scroll container is in the DOM gives it a scroll height of 0, disabling virtualization entirely. Always create it inside onMounted or firstUpdated.
  • Passing a new options object with a different count to a second createVirtualizer call creates a fresh instance, losing scroll position. Use virt.update({ count }) for data changes.
  • Not setting an explicit height on the scroll container makes it expand to fit all rendered rows, removing the scroll and disabling virtualization.