Highlight Code in Node.js

Connect an initialized Ferriki highlighter to Ferromark. The same interface also accepts a compatible synchronous highlighter.

Install and initialize

npm install ferromark ferriki

Create the highlighter once and reuse it. Load the languages and theme your content needs:

import { createHighlighter } from "ferriki";
import { toHtmlWithHighlighter } from "ferromark";

const highlighter = await createHighlighter({
  langs: ["javascript"],
  themes: ["github-dark"],
});

const html = toHtmlWithHighlighter(
  '```javascript\nconsole.log("Hello")\n```',
  highlighter,
  {
    theme: "github-dark",
    onHighlightError(error, { lang }) {
      console.warn(`Could not highlight ${lang}`, error);
    },
  },
  { headingIds: true },
);
console.log(html);

The arguments are Markdown, the highlighter, highlighting settings, and optional Markdown options. Initialization is asynchronous; rendering uses synchronous codeToHtml() calls.

Keep document metadata

import { transformWithHighlighter } from "ferromark";

// Reuse the highlighter initialized above.
const page = transformWithHighlighter(
  "---\ntitle: Example\n---\n# Example",
  highlighter,
  { theme: "github-dark" },
  { frontMatter: true },
);
console.log(page.headings, page.frontMatter);

The callback receives fenced and indented code blocks.

Understand fallback behavior

Unsupported languages and highlighter exceptions fall back to escaped <pre><code> output. onHighlightError can observe exceptions; if it throws, the render call also throws. Invalid highlighter return values surface as native callback errors.

Highlighter HTML is inserted verbatim. The highlighter must escape untrusted code and metadata. Fence metadata after the language is forwarded as meta.__raw for compatible title and line-highlighting transformers.

For the shared trust boundary, see rendering and trust.