UI Components Medium

Code Diff Viewer

Side-by-side code diff viewer with added/removed line highlighting, line numbers, and unified/split toggle. No libraries.

Open in Lab

vanilla-js css

Targets: JS HTML

Code

*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: #0d1117;
  min-height: 100vh;
  padding: 32px 16px;
  display: flex;
  justify-content: center;
}

.demo {
  width: 100%;
  max-width: 900px;
}

.diff-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 16px;
  background: #21262d;
  border-radius: 10px 10px 0 0;
  border: 1px solid #30363d;
  border-bottom: none;
}

.diff-file {
  font-size: 13px;
  font-weight: 600;
  color: #e6edf3;
  font-family: Menlo, monospace;
}

.diff-stats {
  display: flex;
  gap: 10px;
  margin-left: 12px;
}
.stat-add {
  font-size: 12px;
  font-weight: 700;
  color: #3fb950;
}
.stat-del {
  font-size: 12px;
  font-weight: 700;
  color: #f85149;
}

.diff-meta {
  display: flex;
  align-items: center;
}

.view-toggle {
  display: flex;
  background: #30363d;
  border-radius: 8px;
  padding: 2px;
  gap: 2px;
}
.view-btn {
  background: none;
  border: none;
  color: #8b949e;
  font-size: 12px;
  font-weight: 600;
  padding: 4px 12px;
  border-radius: 6px;
  cursor: pointer;
}
.view-btn.active {
  background: #484f58;
  color: #e6edf3;
}

/* Diff container */
.diff-container {
  display: flex;
  border: 1px solid #30363d;
  border-radius: 0 0 10px 10px;
  overflow: hidden;
  background: #0d1117;
}

.diff-container--unified {
  display: block;
}
.diff-container--unified .diff-line-num {
  min-width: 36px;
}

.diff-pane {
  flex: 1;
  min-width: 0;
  overflow-x: auto;
}
.diff-pane-header {
  padding: 8px 16px;
  font-size: 11px;
  font-weight: 700;
  color: #8b949e;
  background: #161b22;
  border-bottom: 1px solid #21262d;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.diff-divider {
  width: 1px;
  background: #21262d;
}

.diff-lines {
  font-family: "JetBrains Mono", Menlo, monospace;
  font-size: 12.5px;
  line-height: 1.6;
}

/* Lines */
.diff-line {
  display: flex;
  align-items: stretch;
  min-height: 24px;
}
.diff-line:hover {
  background: rgba(255, 255, 255, 0.03);
}

.diff-line-num {
  min-width: 44px;
  padding: 0 8px;
  text-align: right;
  color: #4a555f;
  border-right: 1px solid #21262d;
  user-select: none;
  font-size: 11px;
  display: flex;
  align-items: center;
  justify-content: flex-end;
  flex-shrink: 0;
}

.diff-line-sig {
  width: 20px;
  text-align: center;
  font-size: 12px;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.diff-line-text {
  flex: 1;
  padding: 2px 12px 2px 4px;
  color: #e6edf3;
  white-space: pre;
}

/* States */
.diff-line--add {
  background: rgba(63, 185, 80, 0.1);
}
.diff-line--add .diff-line-sig {
  color: #3fb950;
}
.diff-line--del {
  background: rgba(248, 81, 73, 0.1);
}
.diff-line--del .diff-line-sig {
  color: #f85149;
}
.diff-line--empty {
  background: #0d1117;
}
.diff-line--empty .diff-line-text {
  color: transparent;
}
.diff-line--hunk {
  background: rgba(130, 180, 255, 0.06);
}
.diff-line--hunk .diff-line-text {
  color: #79c0ff;
  font-style: italic;
}

// Diff data: each item is [type, leftNum, rightNum, text]
// type: 'ctx' | 'add' | 'del' | 'hunk'
const DIFF = [
  ["hunk", null, null, "@@ -1,12 +1,16 @@"],
  ["ctx", 1, 1, 'import { SignJWT, jwtVerify } from "jose";'],
  ["ctx", 2, 2, 'import { cookies } from "next/headers";'],
  ["ctx", 3, 3, ""],
  ["del", 4, null, "const SECRET = process.env.SECRET;"],
  ["add", null, 4, "const SECRET = new TextEncoder().encode("],
  ["add", null, 5, '  process.env.SECRET ?? ""'],
  ["add", null, 6, ");"],
  ["ctx", 5, 7, ""],
  ["del", 6, null, "export async function createToken(payload) {"],
  ["add", null, 8, "export async function createToken("],
  ["add", null, 9, "  payload: Record<string, unknown>"],
  ["add", null, 10, ") {"],
  ["ctx", 7, 11, "  return new SignJWT(payload)"],
  ["ctx", 8, 12, '    .setProtectedHeader({ alg: "HS256" })'],
  ["ctx", 9, 13, '    .setExpirationTime("7d")'],
  ["del", 10, null, "    .sign(Buffer.from(SECRET));"],
  ["add", null, 14, "    .sign(SECRET);"],
  ["ctx", 11, 15, "}"],
  ["ctx", 12, 16, ""],
];

function makeLine(type, num, text) {
  const sigMap = { add: "+", del: "-", ctx: " ", hunk: "@@" };
  const line = document.createElement("div");
  line.className = "diff-line" + (type !== "ctx" ? ` diff-line--${type}` : "");
  line.innerHTML = `
    <span class="diff-line-num">${num ?? ""}</span>
    <span class="diff-line-sig">${sigMap[type] ?? ""}</span>
    <span class="diff-line-text">${escHtml(text)}</span>
  `;
  return line;
}

function makeEmpty() {
  const line = document.createElement("div");
  line.className = "diff-line diff-line--empty";
  line.innerHTML = `<span class="diff-line-num"></span><span class="diff-line-sig"></span><span class="diff-line-text">~</span>`;
  return line;
}

function escHtml(t) {
  return t.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");
}

function renderSplit() {
  const left = document.getElementById("leftPane");
  const right = document.getElementById("rightPane");
  left.innerHTML = "";
  right.innerHTML = "";

  for (const [type, ln, rn, text] of DIFF) {
    if (type === "hunk") {
      left.appendChild(makeLine("hunk", "...", text));
      right.appendChild(makeLine("hunk", "...", text));
    } else if (type === "del") {
      left.appendChild(makeLine("del", ln, text));
      right.appendChild(makeEmpty());
    } else if (type === "add") {
      left.appendChild(makeEmpty());
      right.appendChild(makeLine("add", rn, text));
    } else {
      left.appendChild(makeLine("ctx", ln, text));
      right.appendChild(makeLine("ctx", rn, text));
    }
  }
}

function makeUnifiedLine(type, oldNum, newNum, text) {
  const sigMap = { add: "+", del: "-", ctx: " ", hunk: "@@" };
  const line = document.createElement("div");
  line.className = "diff-line" + (type !== "ctx" ? ` diff-line--${type}` : "");
  line.innerHTML = `
    <span class="diff-line-num">${oldNum ?? ""}</span>
    <span class="diff-line-num">${newNum ?? ""}</span>
    <span class="diff-line-sig">${sigMap[type] ?? ""}</span>
    <span class="diff-line-text">${escHtml(text)}</span>
  `;
  return line;
}

function renderUnified() {
  const pane = document.getElementById("unifiedPane");
  pane.innerHTML = "";
  for (const [type, ln, rn, text] of DIFF) {
    if (type === "hunk") {
      pane.appendChild(makeUnifiedLine("hunk", "...", "...", text));
    } else if (type === "del") {
      pane.appendChild(makeUnifiedLine("del", ln, "", text));
    } else if (type === "add") {
      pane.appendChild(makeUnifiedLine("add", "", rn, text));
    } else {
      pane.appendChild(makeUnifiedLine("ctx", ln, rn, text));
    }
  }
}

// View toggle
document.querySelectorAll(".view-btn").forEach((btn) => {
  btn.addEventListener("click", () => {
    document.querySelectorAll(".view-btn").forEach((b) => b.classList.remove("active"));
    btn.classList.add("active");
    const v = btn.dataset.view;
    document.getElementById("splitView").hidden = v !== "split";
    document.getElementById("unifiedView").hidden = v !== "unified";
  });
});

renderSplit();
renderUnified();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Diff Viewer</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
<div class="demo">
  <div class="diff-toolbar">
    <div class="diff-meta">
      <span class="diff-file">src/utils/auth.ts</span>
      <span class="diff-stats">
        <span class="stat-add">+8</span>
        <span class="stat-del">-4</span>
      </span>
    </div>
    <div class="view-toggle">
      <button class="view-btn active" data-view="split">Split</button>
      <button class="view-btn" data-view="unified">Unified</button>
    </div>
  </div>

  <!-- Split view -->
  <div class="diff-container" id="splitView">
    <div class="diff-pane">
      <div class="diff-pane-header">Before</div>
      <div class="diff-lines" id="leftPane"></div>
    </div>
    <div class="diff-divider"></div>
    <div class="diff-pane">
      <div class="diff-pane-header">After</div>
      <div class="diff-lines" id="rightPane"></div>
    </div>
  </div>

  <!-- Unified view -->
  <div class="diff-container diff-container--unified" id="unifiedView" hidden>
    <div class="diff-lines" id="unifiedPane"></div>
  </div>
</div>
<script src="script.js"></script>
</body>
</html>

Side-by-side code diff viewer showing added (green) and removed (red) lines with line numbers and change indicators. Includes a toggle between split and unified views. Pure vanilla JS.