Rehype-mermaidjs NPM

rehype-mermaidjs

A rehype plugin to render mermaid diagrams.

Installation

npm install rehype-mermaidjs

In Node.js this package uses playwright under the hood. To use it, you may need to install additional dependencies. These can be installed with:

npx playwright install --with-deps chromium

See the Playwright Browsers documentation for more information.

Usage

This plugin takes all <pre class="mermaid"> and <code class="language-mermaid"> elements, and replaces them with a rendered version of the diagram. If the <code> element is wrapped in a <pre> element, the <pre> element is replaced as well. This is compatible with what Mermaid would render client side, as well as the output of mermaid code blocks processed by remark-rehype.

The plugin has several rendering strategies described below.

Given a file named index.html:

<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <pre><code class="language-mermaid">
      graph TD;
          A-->B;
          A-->C;
          B-->D;
          C-->D;
    </code></pre>
    <pre class="mermaid">
      graph TD;
          A-->B;
          A-->C;
          B-->D;
          C-->D;
    </pre>
  </body>
</html>

The following script:

import { readFile } from 'node:fs/promises'

import { rehype } from 'rehype'
import rehypeMermaid from 'rehype-mermaidjs'

const { value } = await rehype()
  .use(rehypeMermaid, {
    // The default strategy is 'inline-svg'
    // strategy: 'img-png'
    // strategy: 'img-svg'
    // strategy: 'inline-svg'
    // strategy: 'pre-mermaid'
  })
  .process(await readFile('index.html'))

console.log(value)

Yields the following results, depending on the stragey used.

`'img-png'`

This strategy renders a diagram as an <img> element with an inline base64 PNG image. Given the example, this yields:

<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <img alt="" height="215" id="mermaid-0" src="data:image/png;base64,iVBORw0KGgoA…" width="118" />
    <img alt="" height="215" id="mermaid-1" src="data:image/png;base64,iVBORw0KGgoA…" width="118" />
  </body>
</html>

This strategy is asynchronous.

`'img-svg'`

This strategy renders a diagram as an <img> element with an inline SVG image. Given the example, this yields:

<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <img alt="" height="215" id="mermaid-0" src="data:image/xml+svg,%3csvg…" width="118" />
    <img alt="" height="215" id="mermaid-1" src="data:image/xml+svg,%3csvg…" width="118" />
  </body>
</html>

This strategy is asynchronous.

`'inline-svg'`

This strategy renders a diagram as an inline <svg> element. Given the example, this yields:

<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <svg id="mermaid-0" …>…</svg>
    <svg id="mermaid-1" …>…</svg>
  </body>
</html>

This strategy is asynchronous.

`'pre-mermaid'`

This strategy replaces the element with a <pre class="mermaid"> element with only the diagram as its child. Given the example, this yields:

<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <pre class="mermaid">
      graph TD;
          A-->B;
          A-->C;
          B-->D;
          C-->D;
    </pre>
    <pre class="mermaid">
      graph TD;
          A-->B;
          A-->C;
          B-->D;
          C-->D;
    </pre>
  </body>
</html>

This allows Mermaid to render the diagram on the client side, for example using:

import mermaid from 'mermaid'

mermaid.initialize({ startOnLoad: true })

This strategy is synchronous.

API

This package has a default export rehypeMermaid.

`unified().use(rehypeMermaid, options?)`

`options`

`browser`

The Playwright browser to use. (object, default: chromium)

`css`

A URL that points to a custom CSS file to load. Use this to load custom fonts. This option is ignored in the browser. You need to include the CSS in your build manually. (string | URL)

`errorFallback`

Create a fallback node if processing of a mermaid diagram fails. If nothing is returned, the code block is removed. The function receives the following arguments:

element The hast element that could not be rendered.
diagram The Mermaid diagram that could not be rendered as a string.
error: The error that was thrown.
file: The file on which the error occurred.

`launchOptions`

The options used to launch the browser. (object)

`mermaidConfig`

A custom Mermaid configuration. By default fontFamily is set to arial,sans-serif. This option is ignored in the browser. You need to call mermaid.initialize() manually. (object)

`prefix`

A custom prefix to use for Mermaid IDs. (string, default: mermaid)

`strategy`

The render strategy to use. One of 'img-png', 'img-svg', 'inline-svg', or 'pre-mermaid'. (default: 'inline-svg')

Compatibility

This project is compatible with Node.js 16 or greater. It’s compatible with mermaid code blocks processed by remark-rehype. This means it’s also compatible with MDX.

Related Projects

mermaid is the library that’s used to render the diagrams.
mermaid-isomorphic allows this package to render Mermaid diagrams in both Node.js and the browser.
rehype provides HTML processing using a unified pipeline.

Contributing

Test fixtures are generated and verified using Linux. Rendering on other platforms may yield slightly different results. Don’t worry about adding new fixtures, but don’t update existing ones that cause CI to fail. Furthermore see my global contributing guidelines.