ng-injector-helper.js

// Analyzes a contrusctor and creates the array and final function for injection
// e.g.

ServiceInstance.$inject = ['$rootScope', '$q', 'ServiceA', 'ServiceB'];

funciton ServiceInstance($rootScope, $q, Service, ServiceB)
{
  this.fetch = function () {};
  
  // ... whatever logic
}

// from a provider, we would do...

this.$get = ['$rootScope', '$q', 'ServiceA', 'ServiceB', function ($rootScope, $q, ServiceA, ServiceB) {
    return new ServiceInstance($rootScope, $q, Service, ServiceB);
}];

// which is annoying especially when our ServiceInstance already has an $inject property for explicit injection
// let's do this instead!

this.$get = constructService(ServiceInstance);
// when not using injectables argument but instead the service's $inject property,
// only call it after the $inject property is set

// Or we should really just do it the Angular way...
this.$get = ['$injector', function ($injector) {
  // This is the same as doing module.service
  return $injector.instantiate(ServiceInstance);
}];

// Alternatively, if you want to register it as a factory (useful when wanting to "new" a service when being used in a component)

this.$get = ['$rootScope', '$q', 'ServiceA', 'ServiceB', function ($rootScope, $q, Service, ServiceB) {
  function Factory()
  {
    // define properties
  }
  
  Factory.prototype.fetch = function () {
    // fetch logic... e.g.
    
    return $q.when(ServiceB);
  };
  
  return Factory;
}];

/**
 * Constructs a service that can be used in a provider's $get property
 * @param {Function} service
 * @param {Array} [injectables]
 * @returns {Array}
 */
function constructService(service, injectables)
{
    if (Array.isArray(service.$inject)) {
        injectables = angular.copy(service.$inject);
    } else if (Array.isArray(injectables)) {
        // do not mutate original
        injectables = angular.copy(injectables);
    } else {
        injectables = [];
    }

    injectables.push(function () {
        var args = [service];

        for (var a = 0; a < arguments.length; a++) {
            args.push(arguments[a]);
        }

        return new (service.bind.apply(service, args));
    });

    return injectables;
}