index.js

const { read, write } = require("to-vfile");
const remark = require("remark");
const mdx = require("remark-mdx");
const visit = require("unist-util-visit");

const customPlugin = () => (tree, vfile) => {
  vfile.data.plugin = "Set by custom plugin";

  visit(tree, "heading", node => {
    if (node.depth === 1) {
      node.children = [{ type: "text", value: "Set by custom plugin" }];
    }
  });
};

(async () => {
  const inPath = "./example.mdx";
  const outPath = "./example-out.mdx";

  // read input as vfile
  const file = await read(inPath);

  // Remark parser/stringify/processor using MDX plugin
  const processor = remark()
    .use(customPlugin)
    .use(mdx);

  // parse the MDX file into MDXAST
  const tree = processor.parse(file.contents.toString());

  // run any other transformer plugins against the MDXAST
  const result = await processor.run(tree, file);

  // stringify the result (remark-stringify)
  const contents = await processor.stringify(result);

  // write a vfile to disk
  await write(
    Object.assign({}, file, {
      path: outPath,
      contents
    })
  );
})();