index.js

module.exports = shimRequire;

const Module = require('module');
const fs = require('fs');
const extensions = ['js','json'];
const defaultExtension = 'js';

// from https://github.com/nodejs/node/blob/master/lib/internal/modules/cjs/loader.js
function shimRequire(callback, extension) {
  if (extension && !extensions.includes(extension)) {
    throw `Invalid extension ${extension} - supported types ${extensions.join(',')}`;
  }
  if (!extension) {
    extension = defaultExtension;
  }
  Module._extensions[`.${extension}`] = function(module, filename) {
    let content = fs.readFileSync(filename, 'utf8');
    try {
      content = stripBOM(content);
      content = callback(content, filename, module);
      if (extension === 'js') {
        module._compile(content, filename);
      }
      else if (extension === 'json') {
        module.exports = JSON.parse(content);
      }
    } catch (err) {
      err.message = filename + ': ' + err.message;
      throw err;
    }
  };
}

function stripBOM(content) {
  if (content.charCodeAt(0) === 0xFEFF) {
    content = content.slice(1);
  }
  return content;
}