docs/lib/remark-include.js

const path = require('path');
const remark = require('remark');
const flatMap = require('unist-util-flatmap');
const { readSync } = require('to-vfile');

module.exports = function includeMarkdownPlugin({ resolveFrom } = {}) {
  return function transformer(tree, file) {
    return flatMap(tree, node => {
      if (node.type !== 'paragraph') return [node];

      // detect an `@include` statement
      const includeMatch =
        node.children[0].value && node.children[0].value.match(/^@include\s['"](.*)['"]$/);
      if (!includeMatch) return [node];

      // read the file contents
      const includePath = path.join(resolveFrom || file.dirname, includeMatch[1]);
      let includeContents;
      try {
        includeContents = readSync(includePath, 'utf8');
      } catch (err) {
        console.log(err);
        throw new Error(
          `The @include file path at ${includePath} was not found.\n\nInclude Location: ${file.path}:${node.position.start.line}:${node.position.start.column}`
        );
      }

      // if we are including a ".md" or ".mdx" file, we add the contents as processed markdown
      // if any other file type, they are embedded into a code block
      if (includePath.match(/\.md(?:x)?$/)) {
        // return the file contents in place of the @include
        // this takes a couple steps because we allow recursive includes
        const processor = remark().use(includeMarkdownPlugin, { resolveFrom });
        const ast = processor.parse(includeContents);
        return processor.runSync(ast, includeContents).children;
      } else {
        // trim trailing newline
        includeContents.contents = includeContents.contents.trim();

        // return contents wrapped inside a "code" node
        return [
          {
            type: 'code',
            lang: includePath.match(/\.(\w+)$/)[1],
            value: includeContents,
          },
        ];
      }
    });
  };
};