Dependency injection done right.

npm install r42
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Mix in node's require, some angular dependency injection look&feel and the ease of use of RequireJS and what do you get?

Dependency injection done right for Node.js!

Getting started


npm install r42

Your first module

Let's code lib/toto.js :

// path & fs are examples to show you how to load node modules
// _ will contain lodash (see next section to see why)
define(function (path, fs, _, dep1, dep2) {
  /* Your code goes here */

  // You can export anything, a scalar, a function, an object, an array...
  // Objects & functions should be preferred
  return /* what you exports goes here */;

Configuring & lauching your project

var r42 = require('r42');
var path = require('path');

var context = r42.config({
  // Sets the root directory in which modules are looked for
  baseDir: path.join(__dirname, 'lib'),

  // Allows to map paths / module names to something else
  paths: {
    // _ will load lodash
    _: 'lodash',

    // Target paths are all relative to baseDir.
    // Here "library" will try to load "baseDir + '../vendor/library'"
    library: '../vendor/library',

    // Replacement in paths are also possible: here all modules named
    // 'sub/.../truc' will be looked for into the ../sub folder
    sub: '../sub',

    // Alias can also be used in configuration but they HAVE TO be declared
    // BEFORE being used in the object
    shMod: 'sh/module', // refers to 'sh/module'
    sh: 'shortcut',
    shMod2: 'sh/module2', // refers to 'shortcut/module2'

// Let's get started
context.inject(function (toto) {
  // you can use your dependency here

More complex dependencies

Module in subfolders

If you want to resolve complex names in subdirectories, you can use the optional replacer argument of the define function. Here is an example:

  test: 'sub/folder/test',
}, function (test) {
  // here test will be resolved to module 'sub/folder/test'

The object maps an argument's name to the real module name locally. The argument name will be replaced by the given module name before injection happens.

You can also use special r42 comments (looking like /*! module */) before your argument names:

define(function (/*! sub/folder/test */ test) {
  // here test will be resolved to module 'sub/folder/test'

Note : spaces are optional in r42 comments.

Modules in the same folder

Sometimes, it is a pain to refer to a module in the same folder as you are right now. r42 allows for a fine way to do so.

Using $ to prefix your variable's name will automatically cause r42 to replace it by your current module's "path". It also works to prefix files in the replacer object.

In a module module/toto.js:

  superSub: '$super/sub',
}, function (superSub, $sideFn, /*! $sub/sideValue */ sideVal) {
  // superSub refers to 'module/super/sub'
  // $sideFn refers to module/sideFn
  // sideVal refers to $sub/sideValue
Special case : index.js in a subfolder

You can refer to an index module using only the folder's name just like so:

In file folder/index.js:

define(function () {
  return {
    answer: 42

And in a file at the same level as folder:

define(function ($folder) {
  // Here $folder will load folder/index.js
Circular dependencies

Those are working "automatically" but you NEED to exports barebone objects on both modules for it to work.

Here is an example:

In a module a.js:

define(function (b) {
  // here b might be empty if it was loaded before a, so don't use it

  return {
    aFn: function () {
      // here b should be fully usable

In a module b.js:

define(function (a) {
  // here a might be empty if it was loaded before b, so don't use it

  return {
    bFn: function () {
      // here a should be fully usable
Other load mechanism

r42 allows you to create your own loading modules & to load modules using a different policy than its own. By default, a require load mechanism & a json load mechanism are included by default in r42.

To use a different load mechanism than the default one, you need to prefix its name by plugin! (eg: require!module or json!module). This implies that you have to use the replacer argument or r42 comments to load a plugin using a specific loader.


With the json loader:

define(function (/*! json!$config */ config) {
  // Here "config" contains ./config.json parsed into a JS structure

With the require loader:

define(function (/*! require!$externalModule */ externalModule) {
  // Here "externalModule" contains the value returned by calling require with
  // ./externalModule

Special APIs

Loading modules dynamically

You can also use r42.inject to load modules dynamically. In this case, provide a module name or module list and optionally a replacer as usual. Here is what it could look like:

In module dynamic.js

define(function (r42) {
  // This will load lodash in variable _.
  var _ = r42.inject('lodash');
  // Same thing for the variable alias but using the replacer argument.
  var alias = r42.inject({ alias: 'lodash' }, 'alias');

  // An example with multiple load at once
  // Here modules[0] will resolve to require('util')
  // and modules[1] to require('net')
  var modules = r42.inject(['util', 'net']);

  // You can also use the replacer argument with an array:
  // this example will load the same modules as the previous one
  modules = r42.inject({a: 'util', b: 'net'}, ['a', 'b']);
Testing API

There is a test API that is intended to help writing tests & speed up tests by preventing a module to be reloaded more than necessary. This API is accessible when the NODE_ENV environment variable is set to "test" or by passing "test" as a first argument to the createSub method as demonstrated below:

var context = /* create here as you want a r42 context  with normal paths */

var subContext = context.createSub(
  "test", // this argument can be omitted if process.env.NODE_ENV="test"
    paths: {
      moduleToMockup: 'mockupModule',

// now subContext can be used just as context and inherits its configuration
subContext.inject(/* inject as usual */);
Writing your own loader plugin

You can write other loader plugins and register them to r42 to use them after that in your codebase. Here is how to do so:

var r42 = require('r42');
var path = require('path');
var fs = require('fs');
var myYamlModule  = require('myYamlModule');

r42.registerLoader('yaml', function (moduleDef, parent, mc) {
  // moduleDef contains information about the module that should be loaded
  // parent contains information about the module that required moduleDef
  // mc is the context object, it handles the modules' cache

  // mc.$baseDir is the root path of the project
  // moduleDef.name contains the path to the module from baseDir
  // (without any extension)
  var filePath = path.join(mc.$baseDir, moduleDef.name + '.yaml');

  try {
    // The loading should be synchronous
    var yaml = fs.readFileSync(filePath, 'utf8');
    yaml = myYamlModule.parse(yaml);

    // Init the module into the context
    mc.initModule(moduleDef, yaml);
  } catch (e) {
    // <module>.fullName contains the plugin's name + ! + the module's name
    e.message = '[r42:yaml] Cannot load module ' + moduleDef.fullName +
      ' (required by module ' + parent.fullName + '): ' + e.message;
    throw e;
Printing dependencies

This function might be useful to gather information about your module & to debug dependencies problems. It takes an options parameter which is an object that can take the following two options:

  • colors (defaults to true): whether to colorize the output or not
  • print (defaults to true): whether to print to stdout automatically the result or not

WARNING: The colors option requires that you install chalk (version >=0.4.0) as a dependency of your project.

It returns a string containing the dependencies information.

var r42 = require('r42');

var context = r42.config({
  // Your config HERE

// Let's get started
context.inject(function (/* Your dependencies */) {
  // Dump dependencies with colors using process.stdout.print

  // This verion will return you the string without any colors and without printing it :
  var depstr = context.dumpDeps({
    colors: false, // Do not use term colors in the output string
    print: false,   // Do not print the string on stdout
  // Now you can do whatever you want with the string, like send it over network...

Using r42 in libraries

Since version 0.0.21, r42 has been rewritten to be completely synchronous. This means that you can use it easily, even to create library packages. Here is a simple example of what your main library file might look like :

var r42 = require('r42');
var context = r42.config(/* Your configuration object */);

context.inject(function (/* Your dependencies */) {
  module.exports = {
    // Whatever


0.5.0 & 0.5.1

  • Add testing API


  • Add plugin API
  • Add plugins
    • default (previous loading mechanism moved to a plugin)
    • require (force use of require, no define expected in module)
    • json (load JSON file - using require)


  • Remove the beta channel warning that was forgotten


  • Make r42 instantiable.
  • Clean up related require's cache after loading a module with r42


  • Continue work started with 0.2.1 to prepare for 0.3.0


  • Change architecture to prepare for 0.3.0 that will change r42 API so that r42 si not a singleton anymore


  • Colors dependency was dumped in favor of chalk that does not mess up String's prototype
  • dumpDeps
    • small bug fixes
    • better readability
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