meteor/packages/modules-runtime/server.js

// RegExp matching strings that don't start with a `.` or a `/`.
var topLevelIdPattern = /^[^./]/;

// This function will be called whenever a module identifier that hasn't
// been installed is required. For backwards compatibility, and so that we
// can require binary dependencies on the server, we implement the
// fallback in terms of Npm.require.
makeInstallerOptions.fallback = function (id, parentId, error) {
  // For simplicity, we honor only top-level module identifiers here.
  // We could try to honor relative and absolute module identifiers by
  // somehow combining `id` with `dir`, but we'd have to be really careful
  // that the resulting modules were located in a known directory (not
  // some arbitrary location on the file system), and we only really need
  // the fallback for dependencies installed in node_modules directories.
  if (topLevelIdPattern.test(id)) {
    if (typeof Npm === "object" &&
        typeof Npm.require === "function") {
      return Npm.require(id);
    }
  }

  throw error;
};

makeInstallerOptions.fallback.resolve = function (id, parentId, error) {
  if (topLevelIdPattern.test(id)) {
    // Allow any top-level identifier to resolve to itself on the server,
    // so that makeInstallerOptions.fallback has a chance to handle it.
    return id;
  }

  throw error;
};

install = makeInstaller(makeInstallerOptions);
var Module = install.Module;

Module.prototype.useNode = function () {
  if (typeof npmRequire !== "function") {
    // Can't use Node if npmRequire is not defined.
    return false;
  }

  var parts = this.id.split("/");
  var start = 0;
  if (parts[start] === "") ++start;
  if (parts[start] === "node_modules" &&
      parts[start + 1] === "meteor") {
    start += 2;
  }

  if (parts.indexOf("node_modules", start) < 0) {
    // Don't try to use Node for modules that aren't in node_modules
    // directories.
    return false;
  }

  try {
    npmRequire.resolve(this.id);
  } catch (e) {
    return false;
  }

  this.exports = npmRequire(this.id);

  return true;
};