Code Block

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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: #0d1117;
  color: #e6edf3;
  min-height: 100vh;
  padding: 40px 24px;
  display: flex;
  justify-content: center;
}

.demo {
  width: 100%;
  max-width: 680px;
}

.section-label {
  font-size: 11px;
  font-weight: 700;
  color: #8b949e;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 10px;
}

/* Code block */
.code-block {
  background: #161b22;
  border: 1px solid #30363d;
  border-radius: 10px;
  overflow: hidden;
}

.code-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 16px;
  background: #21262d;
  border-bottom: 1px solid #30363d;
}

.code-lang {
  font-size: 12px;
  font-weight: 600;
  color: #8b949e;
}

.copy-btn {
  display: flex;
  align-items: center;
  gap: 5px;
  background: none;
  border: 1px solid #30363d;
  color: #8b949e;
  font-size: 12px;
  padding: 4px 10px;
  border-radius: 6px;
  cursor: pointer;
  transition: color 0.15s, border-color 0.15s, background 0.15s;
}
.copy-btn:hover {
  color: #e6edf3;
  border-color: #8b949e;
  background: #30363d;
}
.copy-btn.copied {
  color: #3fb950;
  border-color: #3fb950;
}

.icon-check {
  display: none;
}
.copy-btn.copied .icon-copy {
  display: none;
}
.copy-btn.copied .icon-check {
  display: block;
}

.code-pre {
  margin: 0;
  padding: 16px;
  overflow-x: auto;
  font-size: 13px;
  line-height: 1.7;
  font-family: "JetBrains Mono", "Fira Code", "Cascadia Code", Menlo, monospace;
  counter-reset: line;
}

.code-body {
  display: block;
}

/* Syntax coloring */
.tok-kw {
  color: #ff7b72;
} /* keyword */
.tok-fn {
  color: #d2a8ff;
} /* function */
.tok-str {
  color: #a5d6ff;
} /* string */
.tok-num {
  color: #79c0ff;
} /* number */
.tok-cmt {
  color: #8b949e;
  font-style: italic;
} /* comment */
.tok-type {
  color: #ffa657;
} /* type/interface */
.tok-prop {
  color: #e6edf3;
} /* property */
.tok-punc {
  color: #8b949e;
} /* punctuation */

/* Line numbers */
.code-block--lines .code-pre {
  padding-left: 0;
}

.code-line {
  display: flex;
}

.code-line-num {
  min-width: 44px;
  padding: 0 12px 0 16px;
  color: #4a555f;
  user-select: none;
  text-align: right;
  flex-shrink: 0;
  counter-increment: line;
}

.code-line-text {
  flex: 1;
  padding-right: 16px;
}

/* Highlighted lines */
.code-line--highlight {
  background: rgba(255, 204, 0, 0.08);
  border-left: 2px solid #e3b341;
  margin-left: -1px;
}
.code-line--highlight .code-line-num {
  color: #e3b341;
}

// Minimal regex tokenizer
function tokenize(code, lang) {
  const rules = [
    { cls: "tok-cmt", re: /\/\/[^\n]*/g },
    { cls: "tok-str", re: /(['"`])(?:(?!\1)[^\\]|\\.)*\1/g },
    {
      cls: "tok-kw",
      re: /\b(async|await|function|return|const|let|var|if|else|throw|new|import|export|interface|type|extends|implements|class|for|of|in)\b/g,
    },
    { cls: "tok-type", re: /\b(Promise|Error|string|number|boolean|void|null|undefined|User)\b/g },
    { cls: "tok-fn", re: /\b([a-zA-Z_$][a-zA-Z0-9_$]*)(?=\s*\()/g },
    { cls: "tok-num", re: /\b\d+(\.\d+)?\b/g },
  ];

  // Escape HTML first
  let safe = code.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");

  const tokens = [];
  for (const { cls, re } of rules) {
    let m;
    re.lastIndex = 0;
    while ((m = re.exec(safe)) !== null) {
      tokens.push({ start: m.index, end: m.index + m[0].length, cls, text: m[0] });
    }
  }

  // Sort and remove overlaps (keep first)
  tokens.sort((a, b) => a.start - b.start);
  const used = tokens.filter((t, i) => i === 0 || t.start >= tokens[i - 1].end);

  // Reconstruct
  let result = "";
  let cursor = 0;
  for (const { start, end, cls, text } of used) {
    result += safe.slice(cursor, start);
    result += `<span class="${cls}">${text}</span>`;
    cursor = end;
  }
  result += safe.slice(cursor);
  return result;
}

function renderBlock(block) {
  const codeEl = block.querySelector(".code-body");
  const raw = codeEl.dataset.raw || "";
  const highlight = new Set(
    (codeEl.dataset.highlight || "")
      .split(",")
      .map((n) => parseInt(n))
      .filter(Boolean)
  );
  const lang = block.dataset.language || "";
  const showLines = block.classList.contains("code-block--lines") || highlight.size > 0;
  const highlighted = tokenize(raw, lang);
  const lines = highlighted.split("\n");

  if (showLines) {
    block.querySelector(".code-pre").style.padding = "8px 0";
    codeEl.innerHTML = lines
      .map((line, i) => {
        const num = i + 1;
        const isHL = highlight.has(num);
        return (
          `<span class="code-line${isHL ? " code-line--highlight" : ""}">` +
          `<span class="code-line-num">${num}</span>` +
          `<span class="code-line-text">${line}</span>` +
          `</span>`
        );
      })
      .join("\n");
  } else {
    codeEl.innerHTML = highlighted;
  }
}

// Copy button
document.querySelectorAll(".copy-btn").forEach((btn) => {
  btn.addEventListener("click", () => {
    const block = btn.closest(".code-block");
    const raw = block.querySelector(".code-body").dataset.raw || "";
    navigator.clipboard.writeText(raw).then(() => {
      btn.classList.add("copied");
      btn.querySelector(".copy-label").textContent = "Copied!";
      setTimeout(() => {
        btn.classList.remove("copied");
        btn.querySelector(".copy-label").textContent = "Copy";
      }, 2000);
    });
  });
});

document.querySelectorAll(".code-block").forEach(renderBlock);

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Code Block</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
<div class="demo">
  <h2 class="section-label">JavaScript</h2>

  <div class="code-block" data-language="JavaScript">
    <div class="code-header">
      <span class="code-lang">JavaScript</span>
      <button class="copy-btn" aria-label="Copy code">
        <svg class="icon-copy" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><rect x="9" y="9" width="13" height="13" rx="2"/><path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1"/></svg>
        <svg class="icon-check" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5"><polyline points="20 6 9 17 4 12"/></svg>
        <span class="copy-label">Copy</span>
      </button>
    </div>
    <pre class="code-pre"><code class="code-body" data-raw="async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error('Not found');
  return res.json();
}

fetchUser(42).then(user => {
  console.log(user.name);
}).catch(console.error);"></code></pre>
  </div>

  <h2 class="section-label" style="margin-top:28px;">With line numbers</h2>

  <div class="code-block code-block--lines" data-language="CSS">
    <div class="code-header">
      <span class="code-lang">CSS</span>
      <button class="copy-btn" aria-label="Copy code">
        <svg class="icon-copy" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><rect x="9" y="9" width="13" height="13" rx="2"/><path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1"/></svg>
        <svg class="icon-check" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5"><polyline points="20 6 9 17 4 12"/></svg>
        <span class="copy-label">Copy</span>
      </button>
    </div>
    <pre class="code-pre"><code class="code-body" data-raw=".container {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 24px;
  padding: 40px;
}

@media (max-width: 600px) {
  .container { padding: 16px; }
}"></code></pre>
  </div>

  <h2 class="section-label" style="margin-top:28px;">With highlighted lines</h2>

  <div class="code-block" data-language="TypeScript">
    <div class="code-header">
      <span class="code-lang">TypeScript</span>
      <button class="copy-btn" aria-label="Copy code">
        <svg class="icon-copy" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><rect x="9" y="9" width="13" height="13" rx="2"/><path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1"/></svg>
        <svg class="icon-check" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5"><polyline points="20 6 9 17 4 12"/></svg>
        <span class="copy-label">Copy</span>
      </button>
    </div>
    <pre class="code-pre"><code class="code-body" data-raw="interface User {
  id: number;
  name: string;
  email: string;
}

function getUser(id: number): Promise&lt;User&gt; {
  return fetch(`/api/users/${id}`).then(r => r.json());
}" data-highlight="7,8"></code></pre>
  </div>
</div>
<script src="script.js"></script>
</body>
</html>