src/evaluator/modules.js
function customRequire(shell, name) {
  var module = shell.requiredModules[name];
  if (module) {
    return module;
  }

  if (_knownModules[name]) {
    module = require(name);
  }
  else if (shell.installedModules[name]) {
    // Directly load up a specified custom module from where it would have been installed
    // when using the %module command.
    var modulePath = path.join(shell.config.userPath, 'node_modules', name);
    module = require(modulePath);
  }

  if (module) {
    shell.requiredModules[name] = module;
  }
  else {
    throw shell.createError('Unknown module "%s". Make sure it has been installed via %module.',
                            name);
  }

  return module;
}
function customRequire(shell, name) {
  var module = shell.requiredModules[name];
  if (module) {
    return module;
  }

  if (_knownModules[name]) {
    module = require(name);
  }
  else if (shell.installedModules[name]) {
    // Directly load up a specified custom module from where it would have been installed
    // when using the %module command.
    var modulePath = path.join(shell.config.userPath, 'node_modules', name);
    module = require(modulePath);
  }

  if (module) {
    shell.requiredModules[name] = module;
  }
  else {
    throw shell.createError('Unknown module "%s". Make sure it has been installed via %module.',
                            name);
  }

  return module;
}
The implementation of the 'require' method that is made available within the shell. This satisfies required modules using a combination of known modules (standard and other modules available via the shell) and custom modules (previously installed using %module).