Docsify Mermaid 10

Using Docsify and Mermaid 10 can be tricky since Mermaid 10 renders asynchronously (mermaid.render returns a Promise).

This post shows how to work around the mismatch between Docsify’s markdown.renderer.code and Mermaid’s render function on Mermaid v9 and v10.

Working example available at: github.com/HugoDF/docsify-mermaid-10.

A summary of the problem

To extend Docsify markdown rendering, a $docsify.markdown.renderer.code function can be provided. However $docsify.markdown.renderer.code expects a function that returns a string, not a Promise.

This causes problems due to Mermaid v9+ rendering. On Mermaid v9, the flowchart-elk diagram is rendered asynchronously (mermaid.render returns a promise). This behaviour of mermaid.render returning a Promise is true for all diagrams in Mermaid v10.

A naive implementation when mermaid.render returns a Promise would be to extract the svg property as follows:

window.$docsify = {
  markdown: {
    renderer: {
      code(code, lang) {
        if (lang === 'mermaid') {
          return mermaid.render(`mermaid-svg`, code).then(({ svg }) => svg);
        }
        return this.origin.code.apply(this, arguments);
      },
    },
  },
};

When we load this in the browser, the block is rendered as follows, this proves the point that $docsify.markdown.renderer.code does not support a function that returns a Promise:

[object Promise]

We’ve now covered the issue caused by the mismatch of interface between mermaid.render (which returns a Promise) and $docsify.markdown.renderer.code (which expects a string). Next we’ll see how to work around this.

A solution

Since we control the return value of $docsify.markdown.renderer.code and the code in that function runs with access to the page’s DOM, we can solve this issue as follows:

1. Docsify starts running markdown.renderer.code
2. Start Mermaid rendering (out of band, we won’t wait for this to complete before doing 3.)
3. Return a string with an element which can be referenced by id (MERMAID_CONTAINER) or other attribute later to Docsify
4. Docsify will render the returned element
5. When Mermaid rendering completes (started in 2.)
   1. Inject the generated SVG into the MERMAID_CONTAINER element

As a timeline, this looks as follows:

We can code this up as follows:

let svgCounter = 0;

window.$docsify = {
  markdown: {
    renderer: {
      code(code, lang) {
        if (lang === 'mermaid') {
          const svgName = `mermaid-svg-${svgCounter++}`;
          const MERMAID_CONTAINER_ID = `${svgName}-container`;

          // 2. start mermaid rendering
          mermaid.render(svgName, code).then(({ svg }) => {
            // 5. On Mermaid render completion, inject the SVG
            // into the element created from 3.
            const containerElement = document.querySelector(
              `#${MERMAID_CONTAINER_ID}`
            );
            if (containerElement) {
              containerElement.innerHTML = svg;
            } else {
              console.error(`Error: #${MERMAID_CONTAINER_ID} not found`);
            }
          });
          // 3. Return an element that can be referenced by mermaid rendering
          return `<div class="mermaid" id="${MERMAID_CONTAINER_ID}"></div>`;
        }
        return this.origin.code.apply(this, arguments);
      },
    },
  },
};

Demos

We can see the code in action at example.codewithhugo.com/docsify-mermaid-10/#/?id=demos.

The source code is available at: github.com/HugoDF/docsify-mermaid-10.