Async service startup for node.js

app-backbone.js

// General app backbone. Start a sequence of services, arranged in dependency order,
// and attach to the app backbone.
const Q = require('q');

// Start the app by completing a sequence of startup steps.
function start( steps ) {
    // Convert steps to list of deferreds.
    steps = steps.map( ( step ) => {
        return ( app ) => {
            // Convert step to list of deferred service steps.
            var services = Object.keys( step )
            .map( ( sname ) => {
                console.log('Binding %s...', sname);
                // Lookup the service binding.
                var service = step[sname];
                // If service is a function then invoke, passing in the app backbone.
                if( typeof service === 'function' ) {
                    service = service( app );
                }
                // Wait for service to complete starting...
                return Q( service )
                .then( ( service ) => {
                    // ...then bind to the backbone under its name.
                    app[sname] = service;
                    return app;
                });
            });
            // Resolve all services for the current step.
            return Q.all( services );
        };
    });
    // Resolve all steps in sequence (to ensure dependency order).
    return steps.reduce( Q.when, Q( {} ) )
}

// Start app services in dependency order
var app = start([
  { settings: loadSettings },
  { db: startDB },
  { http: startHTTP }
]);

// Function to load settings - returns a promise resolving to app config.
function loadSettings() {
  return Q.nfcall( require('fs').loadFile, 'settings.js' )
  .then( ( data ) => {
      return JSON.parse( data.toString() );
  });
}

// Start the DB - return a promise which resolves once the DB is started.
function startDB( app ) {
  // Load DB module and pass it the DB settings.
  return require('dbmodule').start( app.settings.db );
}

function startHTTP( app ) {
  return require('httpmodule').start( app.settings.http );
}