Virtual List

Windowed list rendering pattern for very large datasets using fixed-row virtualization.

vanilla-js css react vue svelte

Targets: TS JS HTML React Vue Svelte

Code

* {
  box-sizing: border-box;
}

:root {
  --bg: #070d17;
  --panel: #121b2b;
  --border: rgba(255, 255, 255, 0.14);
  --text: #e2e8f0;
  --muted: #94a3b8;
  --accent: #38bdf8;
}

body {
  margin: 0;
  font-family: "Space Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
  background: radial-gradient(circle at top, #101e36, var(--bg));
  color: var(--text);
  min-height: 100vh;
}

.shell {
  width: min(760px, calc(100% - 2rem));
  margin: 2rem auto;
}

h1 {
  margin-bottom: 0.35rem;
}

p {
  margin-top: 0;
  color: var(--muted);
}

.meta {
  color: var(--muted);
  font-size: 0.85rem;
  margin-bottom: 0.75rem;
}

.viewport {
  position: relative;
  height: 460px;
  overflow: auto;
  border: 1px solid var(--border);
  border-radius: 14px;
  background: var(--panel);
}

.spacer {
  width: 100%;
}

.rows {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
}

.row {
  height: 44px;
  border-bottom: 1px solid rgba(255, 255, 255, 0.06);
  padding: 0.6rem 0.8rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.row span:last-child {
  color: var(--accent);
  font-size: 0.8rem;
}

(() => {
  const TOTAL = 5000;
  const ROW_HEIGHT = 44;
  const OVERSCAN = 6;

  const DATA = Array.from({ length: TOTAL }, (_, index) => ({
    id: index + 1,
    label: `Record #${index + 1}`,
    group: `Group ${((index % 8) + 1).toString().padStart(2, "0")}`,
  }));

  const viewport = document.getElementById("viewport");
  const spacer = document.getElementById("spacer");
  const rows = document.getElementById("rows");
  const meta = document.getElementById("meta");

  spacer.style.height = `${TOTAL * ROW_HEIGHT}px`;

  const render = () => {
    const scrollTop = viewport.scrollTop;
    const viewHeight = viewport.clientHeight;

    const start = Math.max(0, Math.floor(scrollTop / ROW_HEIGHT) - OVERSCAN);
    const end = Math.min(TOTAL, Math.ceil((scrollTop + viewHeight) / ROW_HEIGHT) + OVERSCAN);

    rows.style.transform = `translateY(${start * ROW_HEIGHT}px)`;

    const fragment = document.createDocumentFragment();
    for (let index = start; index < end; index += 1) {
      const row = DATA[index];
      const el = document.createElement("div");
      el.className = "row";
      el.innerHTML = `<span>${row.label}</span><span>${row.group}</span>`;
      fragment.appendChild(el);
    }

    rows.innerHTML = "";
    rows.appendChild(fragment);
    meta.textContent = `Rendering rows ${start + 1}-${end} of ${TOTAL}`;
  };

  viewport.addEventListener("scroll", render);
  window.addEventListener("resize", render);
  render();
})();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Virtual List</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <main class="shell">
    <header>
      <h1>Virtual List</h1>
      <p>Windowed rendering of 5,000 rows.</p>
    </header>

    <div class="meta" id="meta"></div>

    <section id="viewport" class="viewport" aria-label="Virtualized list" tabindex="0">
      <div id="spacer" class="spacer"></div>
      <div id="rows" class="rows"></div>
    </section>
  </main>

  <script src="script.js"></script>
</body>
</html>

import { useLayoutEffect, useMemo, useRef, useState } from "react";

const TOTAL = 5000;
const ROW_HEIGHT = 44;
const OVERSCAN = 8;

const DATA = Array.from({ length: TOTAL }, (_, index) => ({
  id: index + 1,
  label: `Record #${index + 1}`,
  group: `Group ${((index % 8) + 1).toString().padStart(2, "0")}`,
}));

export default function VirtualListPattern() {
  const viewportRef = useRef<HTMLDivElement | null>(null);
  const [scrollTop, setScrollTop] = useState(0);
  const [viewHeight, setViewHeight] = useState(460);
  const [jumpTo, setJumpTo] = useState("250");

  useLayoutEffect(() => {
    const node = viewportRef.current;
    if (!node) return;

    const updateSize = () => setViewHeight(node.clientHeight);
    updateSize();

    const observer = new ResizeObserver(updateSize);
    observer.observe(node);
    return () => observer.disconnect();
  }, []);

  const windowed = useMemo(() => {
    const start = Math.max(0, Math.floor(scrollTop / ROW_HEIGHT) - OVERSCAN);
    const end = Math.min(TOTAL, Math.ceil((scrollTop + viewHeight) / ROW_HEIGHT) + OVERSCAN);
    return { start, end, rows: DATA.slice(start, end) };
  }, [scrollTop, viewHeight]);

  const jump = () => {
    const value = Number.parseInt(jumpTo, 10);
    if (!Number.isFinite(value)) return;
    const clamped = Math.min(TOTAL, Math.max(1, value));
    const node = viewportRef.current;
    if (!node) return;
    node.scrollTo({ top: (clamped - 1) * ROW_HEIGHT, behavior: "smooth" });
  };

  return (
    <section className="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
      <div className="mx-auto max-w-4xl space-y-4">
        <header className="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
          <p className="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
          <h1 className="mt-1 text-lg font-bold">Virtual List</h1>
          <p className="mt-1 text-sm text-[#8b949e]">
            Windowed rendering keeps a 5,000-row list responsive.
          </p>
        </header>

        <div className="flex flex-wrap items-center gap-2 rounded-xl border border-[#30363d] bg-[#161b22] p-3">
          <label className="text-xs font-semibold text-[#8b949e]">
            Jump to row
            <input
              value={jumpTo}
              onChange={(event) => setJumpTo(event.target.value)}
              className="ml-2 w-24 rounded-md border border-[#30363d] bg-[#0d1117] px-2 py-1 text-xs text-[#e6edf3] outline-none focus:border-[#58a6ff]"
            />
          </label>
          <button
            type="button"
            onClick={jump}
            className="rounded-md border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-3 py-1.5 text-xs font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
          >
            Scroll
          </button>
          <p className="text-xs text-[#8b949e]">
            Rendering rows{" "}
            <span className="font-semibold text-[#e6edf3]">{windowed.start + 1}</span>
            {" - "}
            <span className="font-semibold text-[#e6edf3]">{windowed.end}</span> of{" "}
            <span className="font-semibold text-[#e6edf3]">{TOTAL}</span>
          </p>
        </div>

        <div
          ref={viewportRef}
          className="relative h-[62vh] min-h-[360px] overflow-auto rounded-2xl border border-[#30363d] bg-[#111827]"
          onScroll={(event) => setScrollTop(event.currentTarget.scrollTop)}
        >
          <div style={{ height: TOTAL * ROW_HEIGHT }} />
          <div
            className="absolute left-0 top-0 w-full"
            style={{ transform: `translateY(${windowed.start * ROW_HEIGHT}px)` }}
          >
            {windowed.rows.map((row) => (
              <div
                key={row.id}
                className="grid h-11 grid-cols-[1fr_auto] items-center border-b border-white/5 px-3 text-sm odd:bg-white/[0.01]"
              >
                <div className="flex items-center gap-3">
                  <span className="w-14 font-mono text-xs text-[#8b949e]">#{row.id}</span>
                  <span className="text-[#dce6f2]">{row.label}</span>
                </div>
                <span className="font-mono text-xs text-sky-300">{row.group}</span>
              </div>
            ))}
          </div>
        </div>
      </div>
    </section>
  );
}

<script setup>
import { ref, computed, onMounted, onUnmounted } from "vue";

const TOTAL = 5000;
const ROW_HEIGHT = 44;
const OVERSCAN = 8;

const DATA = Array.from({ length: TOTAL }, (_, index) => ({
  id: index + 1,
  label: `Record #${index + 1}`,
  group: `Group ${String((index % 8) + 1).padStart(2, "0")}`,
}));

const viewportEl = ref(null);
const scrollTop = ref(0);
const viewHeight = ref(460);
const jumpTo = ref("250");
let observer = null;

const startIdx = computed(() => Math.max(0, Math.floor(scrollTop.value / ROW_HEIGHT) - OVERSCAN));
const endIdx = computed(() =>
  Math.min(TOTAL, Math.ceil((scrollTop.value + viewHeight.value) / ROW_HEIGHT) + OVERSCAN)
);
const rows = computed(() => DATA.slice(startIdx.value, endIdx.value));

onMounted(() => {
  if (viewportEl.value) {
    viewHeight.value = viewportEl.value.clientHeight;
    observer = new ResizeObserver(() => {
      viewHeight.value = viewportEl.value.clientHeight;
    });
    observer.observe(viewportEl.value);
  }
});

onUnmounted(() => {
  if (observer) observer.disconnect();
});

function onScroll(e) {
  scrollTop.value = e.currentTarget.scrollTop;
}

function jump() {
  const value = parseInt(jumpTo.value, 10);
  if (!isFinite(value)) return;
  const clamped = Math.min(TOTAL, Math.max(1, value));
  if (viewportEl.value) {
    viewportEl.value.scrollTo({ top: (clamped - 1) * ROW_HEIGHT, behavior: "smooth" });
  }
}
</script>

<template>
  <section class="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
    <div class="mx-auto max-w-4xl space-y-4">
      <header class="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
        <p class="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
        <h1 class="mt-1 text-lg font-bold">Virtual List</h1>
        <p class="mt-1 text-sm text-[#8b949e]">
          Windowed rendering keeps a 5,000-row list responsive.
        </p>
      </header>

      <div class="flex flex-wrap items-center gap-2 rounded-xl border border-[#30363d] bg-[#161b22] p-3">
        <label class="text-xs font-semibold text-[#8b949e]">
          Jump to row
          <input
            v-model="jumpTo"
            class="ml-2 w-24 rounded-md border border-[#30363d] bg-[#0d1117] px-2 py-1 text-xs text-[#e6edf3] outline-none focus:border-[#58a6ff]"
          />
        </label>
        <button
          type="button"
          @click="jump"
          class="rounded-md border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-3 py-1.5 text-xs font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
        >
          Scroll
        </button>
        <p class="text-xs text-[#8b949e]">
          Rendering rows
          <span class="font-semibold text-[#e6edf3]">{{ startIdx + 1 }}</span>
          -
          <span class="font-semibold text-[#e6edf3]">{{ endIdx }}</span> of
          <span class="font-semibold text-[#e6edf3]">{{ TOTAL }}</span>
        </p>
      </div>

      <div
        ref="viewportEl"
        class="relative h-[62vh] min-h-[360px] overflow-auto rounded-2xl border border-[#30363d] bg-[#111827]"
        @scroll="onScroll"
      >
        <div :style="{ height: TOTAL * ROW_HEIGHT + 'px' }" />
        <div
          class="absolute left-0 top-0 w-full"
          :style="{ transform: `translateY(${startIdx * ROW_HEIGHT}px)` }"
        >
          <div
            v-for="row in rows"
            :key="row.id"
            class="grid h-11 grid-cols-[1fr_auto] items-center border-b border-white/5 px-3 text-sm odd:bg-white/[0.01]"
          >
            <div class="flex items-center gap-3">
              <span class="w-14 font-mono text-xs text-[#8b949e]">#{{ row.id }}</span>
              <span class="text-[#dce6f2]">{{ row.label }}</span>
            </div>
            <span class="font-mono text-xs text-sky-300">{{ row.group }}</span>
          </div>
        </div>
      </div>
    </div>
  </section>
</template>

<script>
import { onMount, onDestroy } from "svelte";

const TOTAL = 5000;
const ROW_HEIGHT = 44;
const OVERSCAN = 8;

const DATA = Array.from({ length: TOTAL }, (_, index) => ({
  id: index + 1,
  label: `Record #${index + 1}`,
  group: `Group ${String((index % 8) + 1).padStart(2, "0")}`,
}));

let viewportEl;
let scrollTop = 0;
let viewHeight = 460;
let jumpTo = "250";
let observer;

$: startIdx = Math.max(0, Math.floor(scrollTop / ROW_HEIGHT) - OVERSCAN);
$: endIdx = Math.min(TOTAL, Math.ceil((scrollTop + viewHeight) / ROW_HEIGHT) + OVERSCAN);
$: rows = DATA.slice(startIdx, endIdx);

onMount(() => {
  if (viewportEl) {
    viewHeight = viewportEl.clientHeight;
    observer = new ResizeObserver(() => {
      viewHeight = viewportEl.clientHeight;
    });
    observer.observe(viewportEl);
  }
});

onDestroy(() => {
  if (observer) observer.disconnect();
});

function onScroll(e) {
  scrollTop = e.currentTarget.scrollTop;
}

function jump() {
  const value = parseInt(jumpTo, 10);
  if (!isFinite(value)) return;
  const clamped = Math.min(TOTAL, Math.max(1, value));
  if (viewportEl) {
    viewportEl.scrollTo({ top: (clamped - 1) * ROW_HEIGHT, behavior: "smooth" });
  }
}
</script>

<section class="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
  <div class="mx-auto max-w-4xl space-y-4">
    <header class="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
      <p class="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
      <h1 class="mt-1 text-lg font-bold">Virtual List</h1>
      <p class="mt-1 text-sm text-[#8b949e]">
        Windowed rendering keeps a 5,000-row list responsive.
      </p>
    </header>

    <div class="flex flex-wrap items-center gap-2 rounded-xl border border-[#30363d] bg-[#161b22] p-3">
      <label class="text-xs font-semibold text-[#8b949e]">
        Jump to row
        <input
          bind:value={jumpTo}
          class="ml-2 w-24 rounded-md border border-[#30363d] bg-[#0d1117] px-2 py-1 text-xs text-[#e6edf3] outline-none focus:border-[#58a6ff]"
        />
      </label>
      <button
        type="button"
        on:click={jump}
        class="rounded-md border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-3 py-1.5 text-xs font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
      >
        Scroll
      </button>
      <p class="text-xs text-[#8b949e]">
        Rendering rows
        <span class="font-semibold text-[#e6edf3]">{startIdx + 1}</span>
        {" - "}
        <span class="font-semibold text-[#e6edf3]">{endIdx}</span> of
        <span class="font-semibold text-[#e6edf3]">{TOTAL}</span>
      </p>
    </div>

    <div
      bind:this={viewportEl}
      class="relative h-[62vh] min-h-[360px] overflow-auto rounded-2xl border border-[#30363d] bg-[#111827]"
      on:scroll={onScroll}
    >
      <div style="height: {TOTAL * ROW_HEIGHT}px;" />
      <div
        class="absolute left-0 top-0 w-full"
        style="transform: translateY({startIdx * ROW_HEIGHT}px);"
      >
        {#each rows as row (row.id)}
          <div
            class="grid h-11 grid-cols-[1fr_auto] items-center border-b border-white/5 px-3 text-sm odd:bg-white/[0.01]"
          >
            <div class="flex items-center gap-3">
              <span class="w-14 font-mono text-xs text-[#8b949e]">#{row.id}</span>
              <span class="text-[#dce6f2]">{row.label}</span>
            </div>
            <span class="font-mono text-xs text-sky-300">{row.group}</span>
          </div>
        {/each}
      </div>
    </div>
  </div>
</section>

Virtual List

A high-performance rendering pattern for thousands of rows where only visible items are mounted.

Features

Fixed-height row virtualization
Overscan buffer for smooth scrolling
Absolute-positioned windowed rows
Row index and metadata rendering