Script checking that all links with references are present in Markdown file.

check-markdown-links.js

'use strict';

// Usage
//   node check-markdown-links.js path/to/markdown/file.md

// What it does:
// - Warns when `[ref][]` or `[title][ref]` style links don't have references defined on a page
// - Warns when `[ref]: URL` reference is not used anywhere
// - Warns when two `[ref]: URL` references point to same `URL`
// - Warns when `[ref]: URL` reference is defined in non-lexical order

const fs = require('fs');
const path = require('path');
const filePath = path.resolve(process.cwd(), process.argv[2]);
const lines = fs.readFileSync(filePath, 'utf-8').split(/\n\r?/);

const linkMap = new Map();
const links = [];

let previousTitle = null;
for (let l = 0; l < lines.length; l++) {
  const line = lines[l].trim();
  if (!line) {
    continue;
  }
  else if (line[0] === '#') {
    continue;
  }

  let pos = line.indexOf('[');
  while (line[pos] === '[') {
    let end;
    if (line[pos + 1] === '`') {
      const codeEnd = line.indexOf('`', pos + 2);
      end = line.indexOf(']', codeEnd);
    }
    else {
      end = line.indexOf(']', pos);
    }

    if (end === -1) {
      break;
    }

    const title = line.substring(pos + 1, end);

    if (line[end + 1] === '[') {
      // [link][...]
      pos = end + 1;
      end = line.indexOf(']', pos);

      if (end === pos + 1) {
        // [link][]
        if (!linkMap.get(title)) {
          linkMap.set(title, [{ line: l + 1 }]);
        }
        else {
          linkMap.get(title).push({ line: l + 1 });
        }
      }
      else if (end > pos + 1) {
        // [link][ref]
        const ref = line.substring(pos + 1, end);

        if (!linkMap.get(ref)) {
          linkMap.set(ref, [{ original: title, line: l + 1 }]);
        }
        else {
          linkMap.get(ref).push({ original: title, line: l + 1 });
        }
      }
    }
    else if (line[end + 1] === ':') {
      if (previousTitle && title.toLowerCase() < previousTitle.toLowerCase()) {
        console.warn(`Link ${title} is defined in non-lexical order`);
      }
      previousTitle = title;
      links[title] = line.substring(end + 2).trim();
    }

    pos = line.indexOf('[', end);
  }
}


const keys = Object.keys(links);
for (let o = 0; o < keys.length; o++) {
  const keyO = keys[o];
  const url = links[keyO];
  for (let i = o + 1; i < keys.length; i++) {
    const keyI = keys[i];

    if (url === links[keyI]) {
      console.warn(`Duplicate URL ${url} in links:`);
      console.warn(`  ${keyO}`);
      console.warn(`  ${keyI}`);
    }
  }
}

for (const key of linkMap.keys()) {
  const occurrences = linkMap.get(key);

  if (!links.hasOwnProperty(key)) {
    console.error(`No link with id \`${key}\`. Occurrences:`);
    occurrences.forEach((o) => {
      let out = `  line: ${o.line}`;
      if (o.original) {
        out += ` (original title: ${o.original})`;
      }
      console.error(out);
    });
  }
  else {
    linkMap.delete(key);
    delete links[key];
  }
}

Object.keys(links).forEach((link) => {
  console.warn(`Unused link ${link}`);
});