gist-toc.mjs

#!/usr/bin/env node

// Usage:
// cat my-file.md | ./gist-toc.mjs
// pbpaste | ./gist-toc.mjs | pbcopy # clipboard -> clipboard (macOS)

function createSlug(str) {
  str = str.replace(/[?%:`]/g, '');
  str = str.replace(/ +/g, '-');
  return str.toLowerCase();
}

function createToc(text) {
  let lines = text.trim().split(/\r?\n/);
  lines = lines.filter(line => line.startsWith('##'));
  lines = lines.map((line) => {
    const match = /^(?<prefix>#{2,})\s+(?<title>.*)$/.exec(line);
    if (match && match.groups) {
      const {prefix, title} = match.groups;
      const indent = '  '.repeat(prefix.length-2);
      return indent + `* [${title}](#${createSlug(title)})`;
    } else {
      return 'ERROR: ' + line;
    }
  });
  return lines.join('\n') + '\n';
}

async function main() {
  let text = '';
  for await (const chunk of process.stdin) {
    text += chunk.toString('utf-8');
  }
  const toc = createToc(text);
  text = text.replace('[[toc]]\n', toc);
  console.log(text);
}
main();