footnotes.js

const { readFileSync, writeFileSync } = require('fs');

const re = /\[.*?\](\(.*?\))/g

const addFootnotes = (path, output, ignoreRegExps = []) => {
  let text = readFileSync(path).toString();

  let match;
  let current = 1;
  const matches = [];
  const links = {};
  while (match = re.exec(text)) {
    const link = match[1].substring(1, match[1].length - 1);
    if (ignoreRegExps.reduce((a, c) => a || c.test(link), false)) {
      continue;
    }
    if (links[link] === undefined) {
      links[link] = current++;
    }
    matches.push({ start: match.index, str: match[0], idx: current - 1 });
  }

  let linksList = [];
  Object.keys(links).forEach(l => linksList[links[l] - 1] = l);
  const notes = linksList.map((l, idx) => `${idx + 1}. [###] ${l}`).join('\n');

  for (let i = matches.length - 1; i >= 0; i--) {
    const c = matches[i];
    text = text.substr(0, c.start + c.str.length) + '<sup>' + c.idx + '</sup>' + text.substr(c.start + c.str.length, text.length)
  }

  const result = text + '\n\n' + notes + '\n';
  return writeFileSync(output, result);
};

module.exports.addFootnotes = addFootnotes;