Node.js script to merge markdown files and processing it. Using node.js streams.

markdown-merge.js

const fs = require("fs");
const { Transform, PassThrough } = require('stream');

function concatStreams(streams) {
  let pass = new PassThrough();
  let waiting = streams.length;
  for (let stream of streams) {
      pass = stream.pipe(pass, {end: false});
      stream.once('end', () => --waiting === 0 && pass.emit('end'));
  }
  return pass;
}

class AddEndOfLine extends Transform {
  constructor(options) {
    super(options);
  }
  _transform(data, encoding, callback) {
    this.push(data);

    this.push("\n\n");

    callback();
  }
}

// Convert
//  ![test1](./myImage1.png)
// to
//  ![image](/download/attachments/37064693/myImage1.png)
class RebaseImages extends Transform {
  constructor(options) {
    super(options);
  }
  _transform(data, encoding, callback) {
    const pattern = /!\[.+\]\(\.(\/.+\.png)\)/g;
    const replace = "![image](/download/attachments/37064693$1)";

    const newData = data.toString()
      .replace(pattern, replace);

    this.push(newData);

    callback();
  }
}

// Convert
//  [a b c](./b.md)
// to
//  [a b c](#doc-abc)
class ConvertLinksToAnchors extends Transform {
  constructor(options) {
    super(options);
  }
  _transform(data, encoding, callback) {
    const pattern = /\[(.+)\]\(\.\/(.+\.md)\)/g;

    const newData = data.toString()
      .replace(pattern, (m, p1, p2) => {
        const anchor = p1.replace(/\s/g, "");
        return `[${p1}](#doc-${anchor})`;
      });

    this.push(newData);

    callback();
  }
}

const inPaths = [
  "./examples/a.md",
  "./examples/b.md",
  "./examples/c.md"
];
const outPath = "./examples/out.md";

const inputs = inPaths.map(x => fs.createReadStream(x));
const output = fs.createWriteStream(outPath);

concatStreams(inputs)
  .pipe(new AddEndOfLine())
  .pipe(new RebaseImages())
  .pipe(new ConvertLinksToAnchors())
  .pipe(output)
  .on("finish", function () {
    console.log("Done merging!");
  });