Enable async render operations on Remarkable

async_remarkable.js

// Snippets from my build.js script for my static site generator. Shows how I handle async render operations
// during the Remarkable render operation.
//
const md = new Remarkable("full", markdownOptions).use(katexPlugin).use(require("./consoleFence.js"));

// My own custom codeFence processing for static highlighting using Prism.
md.renderer.rules.fence = require("./codeFence.js");
md.renderer.rules.fence_custom.graph = require("./graphFence.js");
md.renderer.promises = {};
md.renderer.addPromise = (key, promise) => {
  const placeholder = '@+@' + key + '@+@';
  md.renderer.promises[placeholder] = promise;
  return placeholder;
}

const processMarkdown = (files, metalsmith, done) => {
  Object.keys(files).forEach(file => {
    const data = files[file];
    const dirName = path.dirname(file);
    const htmlName = path.basename(file, path.extname(file)) + '.html';
    const htmlPath = dirName !== '.' ? path.join(dirName, htmlName) : htmlName;

    let contents = md.render(data.contents.toString());
    for (let [placeholder, promise] of Object.entries(md.renderer.promises)) {
      promise.then(value => {
        contents = contents.replace(placeholder, value);
        return value;
      });
    }
    
    let allPromise = Promise.all(Object.values(md.renderer.promises));
    allPromise.then(value => {
      data.contents = Buffer.from(contents);
      delete files[file];
      files[htmlPath] = data;
      done();
    });
  });
};