1.4.1 • Published 2 years ago

trapezo v1.4.1

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2 years ago

Trapezo: Node.js dependency injection

"Best use case: Who wants to register relative paths when writing those require statements? Or who likes breaking the require paths when moving a file? I don't - so I wrote Trapezo."

Trapezo is the dependency injection framework written by Jérémie Mercier (Jem) and is based on Wiretree. It goes a step further by adding seamless and flexible management of the dependencies - read more. Follow Jem's projects on twitter.

Working example / project stub

You may begin with downloading a full working project stub with token authentication, routes, solid folder structure, Mongoose, ES6, Mocha unit tests : See Trapezo-Express server or the derived authentication micro-service.

Start developping

Begin with npm install trapezo --save.

Project structure

Let's create the following folder structure and describe each file in subsequent chapters.

    |_ ...


It's highly important to place trapezo.json at the root of your project. It allows detecting the root folder of your app. Trapezo would otherwise fail to start: "No trapezo.json found - reached filesystem root."

I came up with using a plain .json to prevent breaking Mocha and PM2 without using environment variables nor complicated regex heuristics. Pro&cons of alternate methods are discussed here.

Contents of trapezo.json { "root" : true }

Let's now start a trapezo-enabled project and write index.js:

require('trapezo').resolve(module, function(config){
    console.log('server port: ' + config.server.port);

Human friendlier: "Trapezo, browse my files and resolves the dependencies. Then, run this callback".

Note to ES6 developpers the callback cannot (yet) be replaced with an arrow function.

Now, how's that "config" dependency obtained? Read on.

Registering dependencies

Those .tz.js files do the trick:

  • require("trapezo").resolve(...) will browse the files and folders of your project (from the root, remember that trapezo.json file?)
  • each .tz.js runs the local requirestatements and will map the factories to organic dependency names.

Take ./config/config.tz.js:

module.exports = function configure(injector) {
    injector.register('config', require('./config'));
    injector.register('configureExpress', require('./express'));

Full working example of config.js and express.js are available in the project stub Trapezo-Express.

Let's create a mock config.js file to show how a plain object factory is written:

module.exports = function configFactory(/*dependencies go here*/){

    let path    = require('path');
    let address = process.env.IP || "";
    let port = process.env.PORT || 8086;

    return {
        "server" : {
			"port"      : port,
			"address"   : address

Ah. Notice the comment saying "dependencies go here". We'll have an example right after the following .tz.js file that shows a factory with dependencies of its own. For now, we just created the dependency mapper (.tz.js) and its related factory. Factories only run once and that's when its own dependecies were resolved during the .resolve(...) phase.

Now you can move and rename the config subfolder as you please, the dependency will not break.

Let's now inspect ./app/app.tz:

module.exports = function configure(injector) {
    injector.register('main', require('./main'));
    injector.register('routes', require('./routes'));
    injector.register('notFoundHandler', require('./handlers/404'));

Again, I won't clutter this readme with extensive description of routes and 404 handlers from the real life. The project stub Trapezo-Express will help you in that regard.

Let's first modify our index.js to have a working example:

require('trapezo').resolve(module, function(main){
    main.start().then( httpServer => {
        console.log('API: ' + httpServer.address().address + ':' + httpServer.address().port);

Notice that we now ask for the dependency called "main" and expect it to expose a "start" method that returns a promise, passing in an http server (Express) as it resolves.

A quick look at ./app/main.js

module.exports = function mainFactory(config, routes){

    let express = require('express');
	let app = express();
	let httpServer;

    app.use('/api', routes);

    return {
        start: function(){
            return new Promise( (resolve, reject) => {
                httpServer = app.listen(
                                () => resolve(httpServer));

Human friendly:

  • Expect the "config" and "routes" dependency from the injector before running the factory.
  • tell the express app to map the router present in the "route" dependency (described later)
  • A facade is then returned (to be registered by Trapezo) and only contains one method: "start".
  • Start() returns a Promise; the Express httpServer is passed as it resolves.
  • Remember index.js, it requires the "main" dependency and thus can call the "start" method we just described.

The route dependency - see ./app/routes.js

module.exports = function apiRouteFactory(notFoundHandler){

    let express	= require('express');
    let router	= express.Router();

    // Debug route
    router.get('/', (req, res) => res.send('Hello, welcome to server API.') );

        Your routes

    // 404 route

    return router;

The notFoundHandler dependency - see ./app/handlers/404.js

module.exports = function apiRouteFactory(/*dependencies go here*/){
    function fourOhFourHandler(req, res, next) {
	    if (req.accepts('html')) return res.redirect('/404.html'); // Static page
        else if(req.accepts('json')) return res.send({ error: 'resource not found' });
		else res.type('txt').send('Not found');
    return fourOhFourHandler;


Now, you've seen how to register dependencies of various types as both objects and methods: handlers, middlewares, routers, plain key-value pairs and more.

Now, you may want to start developping. Again, have a look at the project stub Trapezo-Express and how 50+ unit tests are written using Trapezo in a non-intrusive manner.

Unit testing

I'm using Mocha and Chai (with chaiHttp) for both behavior driven, test-driven development, API testing and end-to-end testing. Again, have a look at the project stub Trapezo-Express and how 50+ unit tests are written using Trapezo in a non-intrusive manner.

Here's only a wee example of how to use trapezo in your unit testing. See ./test/test-with-di.js

describe('Token revoke functionals', () => {

	let trapezo	= require("trapezo");

	before( done => done() );
	after( done => done() );

	describe("Token revokation list", done => {

		it("is refreshed when a user authenticates", done => {

			// TODO write code that happens before DI resolution:
			// Database connection, mock API, etc.

			trapezo.resolve(module, function(RevokeSchema, revokeList){

			    // TODO write code that after before DI resolution
			    // i.e. logic that mimics interaction with your app/services at runtime

				done(/*pass errors if any*/);


Asynchronous dependencies

Say you have an autoInit database dependency - one that only resolves after the database connection has been established. Trapezo is built on Wirteree and that one library offers an awesome async resolution mechanism: wtDone. Again, back to our Trapezo-Express project stub, we open the following file: ./app/database/db-service-auto-connect.js

module.exports = function dbServiceFactory(wtDone, dbService){
    dbService.connect().then( () => wtDone(dbService) ).catch(err => { console.log(err) });

The database service offers a "connect" method which returns a Promise. As it resolves, you can rely on a valid Mongoose/Mongo connection.


  • version 1.1.0 : add requirement of a "trapezo.json" file to locate root, unit testing and minor fixes.
  • prior to 1.1.0 : initial version. Based on community feedback, used in at least 6 projects between 2016 and 2017. Thanks for using Trapezo!

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