Extract JavaScript fragments from a markdown file

md.extract-js.js

'use strict';

const fs = require('fs');
const path = require('path');
const rl = require('readline');

const [, , mdFilePath] = process.argv;

const jsDirPath = path.join(
  path.dirname(mdFilePath),
  `${path.basename(mdFilePath)}.extracted-js`
);
fs.mkdirSync(jsDirPath);

const rli = rl.createInterface({
  input: fs.createReadStream(mdFilePath, 'utf8'),
});

const jsFragments = [];
const jsFragmentStart = /^(\s*)```(?:js|javascript)\s*$/i;
const jsFragmentEnd   = /^\s*```\s*$/;
let   lineNumber = 0;
let   inJsFragment = false;
let   initialIndentation;

rli.on('line', (line) => {
  lineNumber++;

  if (jsFragmentStart.test(line) && !inJsFragment) {
    inJsFragment = true;
    initialIndentation = new RegExp(`^${line.match(jsFragmentStart)[1]}`);
    jsFragments.push({ startLine: lineNumber + 1, endLine: undefined, js: '' });
  } else if (jsFragmentEnd.test(line) && inJsFragment) {
    inJsFragment = false;
    jsFragments[jsFragments.length - 1].endLine = lineNumber - 1;
  } else if (inJsFragment) {
    jsFragments[jsFragments.length - 1].js += `${line.replace(initialIndentation, '')}\n`;
  }
}).on('close', () => {
  const lastEndLineNumber = jsFragments[jsFragments.length - 1].endLine;

  jsFragments.forEach((jsFragment, i) => {
    jsFragment.startLine = padLeft(jsFragment.startLine, lastEndLineNumber);
    jsFragment.endLine = padLeft(jsFragment.endLine, lastEndLineNumber);

    const jsFileName = `${
      padLeft(i + 1, jsFragments.length)
    }.${jsFragment.startLine}-${jsFragment.endLine}.js`;

    fs.writeFileSync(
      path.join(jsDirPath, jsFileName),

`\uFEFF'use strict';

// ${
  jsFragment.startLine === jsFragment.endLine ?
    jsFragment.startLine :
    `${jsFragment.startLine} – ${jsFragment.endLine}`
}

${jsFragment.js}`,

      'utf8'
    );
  });
});

function padLeft(padNumber, edgeNumber) {
  return `${
    '0'.repeat(String(edgeNumber).length - String(padNumber).length)
  }${padNumber}`;
}

Usage: md.extract-js.js markdown-file.md.

Extracted .js files will be created in the markdown-file.md.extracted-js directory next to markdown-file.md. Each file has this name template: numberOfFragment.numberOfFirstLine-numberOfLastLine.js.