module-loader.js

function loadModule(filename, module, require) {
  const wrappedSrc = `
    function(module, exports, require) {
      ${fs.readFileSync(filename, 'utf8')}
    }(module, module.exports, require)
  `;
   eval(wrappedSrc);
}

require.js

function require(moduleName) {
  const id = require.resolve(moduleName);
  
  if (require.cache[id]) {
    return require.cache[id].exports;
  }

  const module = {
    exports: {},
    id
  }
}

require.cache = {};
require.resolve = (moduleName) => {
  // resolve module based on name
}