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Each web application you build is composed of objects that collaborate to get stuff done. These objects need to be instantiated and wired together for the app to work. In Angular apps most of these objects are instantiated and wired together automatically by the injector service.

The injector creates two types of objects, services and specialized objects.

Services are objects whose API is defined by the developer writing the service.

Specialized objects conform to a specific Angular framework API. These objects are one of controllers, directives, filters or animations.

The injector needs to know how to create these objects. You tell it by registering a "recipe" for creating your object with the injector. There are five recipe types.

The most verbose, but also the most comprehensive one is a Provider recipe. The remaining four recipe types — Value, Factory, Service and Constant — are just syntactic sugar on top of a provider recipe.

Let's take a look at the different scenarios for creating and using services via various recipe types. We'll start with the simplest case possible where various places in your code need a shared string and we'll accomplish this via Value recipe.

Note: A Word on Modules

In order for the injector to know how to create and wire together all of these objects, it needs a registry of "recipes". Each recipe has an identifier of the object and the description of how to create this object.

Each recipe belongs to an Angular module. An Angular module is a bag that holds one or more recipes. And since manually keeping track of module dependencies is no fun, a module can contain information about dependencies on other modules as well.

When an Angular application starts with a given application module, Angular creates a new instance of injector, which in turn creates a registry of recipes as a union of all recipes defined in the core "ng" module, application module and its dependencies. The injector then consults the recipe registry when it needs to create an object for your application.

Value Recipe

Let's say that we want to have a very simple service called "clientId" that provides a string representing an authentication id used for some remote API. You would define it like this:

var myApp = angular.module('myApp', []);
myApp.value('clientId', 'a12345654321x');

Notice how we created an Angular module called myApp, and specified that this module definition contains a "recipe" for constructing the clientId service, which is a simple string in this case.

And this is how you would display it via Angular's data-binding:

myApp.controller('DemoController', ['clientId', function DemoController(clientId) {
  this.clientId = clientId;
}]);
<html ng-app="myApp">
  <body ng-controller="DemoController as demo">
    Client ID: {{demo.clientId}}
  </body>
</html>

In this example, we've used the Value recipe to define the value to provide when DemoController asks for the service with id "clientId".

On to more complex examples!

Factory Recipe

The Value recipe is very simple to write, but lacks some important features we often need when creating services. Let's now look at the Value recipe's more powerful sibling, the Factory. The Factory recipe adds the following abilities:

  • ability to use other services (have dependencies)
  • service initialization
  • delayed/lazy initialization

The Factory recipe constructs a new service using a function with zero or more arguments (these are dependencies on other services). The return value of this function is the service instance created by this recipe.

Note: All services in Angular are singletons. That means that the injector uses each recipe at most once to create the object. The injector then caches the reference for all future needs.

Since a Factory is a more powerful version of the Value recipe, the same service can be constructed with it. Using our previous clientId Value recipe example, we can rewrite it as a Factory recipe like this:

myApp.factory('clientId', function clientIdFactory() {
  return 'a12345654321x';
});

But given that the token is just a string literal, sticking with the Value recipe is still more appropriate as it makes the code easier to follow.

Let's say, however, that we would also like to create a service that computes a token used for authentication against a remote API. This token will be called apiToken and will be computed based on the clientId value and a secret stored in the browser's local storage:

myApp.factory('apiToken', ['clientId', function apiTokenFactory(clientId) {
  var encrypt = function(data1, data2) {
    // NSA-proof encryption algorithm:
    return (data1 + ':' + data2).toUpperCase();
  };

  var secret = window.localStorage.getItem('myApp.secret');
  var apiToken = encrypt(clientId, secret);

  return apiToken;
}]);

In the code above, we see how the apiToken service is defined via the Factory recipe that depends on the clientId service. The factory service then uses NSA-proof encryption to produce an authentication token.

Best Practice: name the factory functions as <serviceId>Factory (e.g., apiTokenFactory). While this naming convention is not required, it helps when navigating the codebase or looking at stack traces in the debugger.

Just like with the Value recipe, the Factory recipe can create a service of any type, whether it be a primitive, object literal, function, or even an instance of a custom type.

Service Recipe

JavaScript developers often use custom types to write object-oriented code. Let's explore how we could launch a unicorn into space via our unicornLauncher service which is an instance of a custom type:

function UnicornLauncher(apiToken) {

  this.launchedCount = 0;
  this.launch = function() {
    // Make a request to the remote API and include the apiToken
    ...
    this.launchedCount++;
  }
}

We are now ready to launch unicorns, but notice that UnicornLauncher depends on our apiToken. We can satisfy this dependency on apiToken using the Factory recipe:

myApp.factory('unicornLauncher', ["apiToken", function(apiToken) {
  return new UnicornLauncher(apiToken);
}]);

This is, however, exactly the use-case that the Service recipe is the most suitable for.

The Service recipe produces a service just like the Value or Factory recipes, but it does so by invoking a constructor with the new operator. The constructor can take zero or more arguments, which represent dependencies needed by the instance of this type.

Note: Service recipes follow a design pattern called constructor injection.

Since we already have a constructor for our UnicornLauncher type, we can replace the Factory recipe above with a Service recipe like this:

myApp.service('unicornLauncher', ["apiToken", UnicornLauncher]);

Much simpler!

Note: Yes, we have called one of our service recipes 'Service'. We regret this and know that we'll be somehow punished for our misdeed. It's like we named one of our offspring 'Child'. Boy, that would mess with the teachers.

Provider Recipe

As already mentioned in the intro, the Provider recipe is the core recipe type and all the other recipe types are just syntactic sugar on top of it. It is the most verbose recipe with the most abilities, but for most services it's overkill.

The Provider recipe is syntactically defined as a custom type that implements a $get method. This method is a factory function just like the one we use in the Factory recipe. In fact, if you define a Factory recipe, an empty Provider type with the $get method set to your factory function is automatically created under the hood.

You should use the Provider recipe only when you want to expose an API for application-wide configuration that must be made before the application starts. This is usually interesting only for reusable services whose behavior might need to vary slightly between applications.

Let's say that our unicornLauncher service is so awesome that many apps use it. By default the launcher shoots unicorns into space without any protective shielding. But on some planets the atmosphere is so thick that we must wrap every unicorn in tinfoil before sending it on its intergalactic trip, otherwise they would burn while passing through the atmosphere. It would then be great if we could configure the launcher to use the tinfoil shielding for each launch in apps that need it. We can make it configurable like so:

myApp.provider('unicornLauncher', function UnicornLauncherProvider() {
  var useTinfoilShielding = false;

  this.useTinfoilShielding = function(value) {
    useTinfoilShielding = !!value;
  };

  this.$get = ["apiToken", function unicornLauncherFactory(apiToken) {

    // let's assume that the UnicornLauncher constructor was also changed to
    // accept and use the useTinfoilShielding argument
    return new UnicornLauncher(apiToken, useTinfoilShielding);
  }];
});

To turn the tinfoil shielding on in our app, we need to create a config function via the module API and have the UnicornLauncherProvider injected into it:

myApp.config(["unicornLauncherProvider", function(unicornLauncherProvider) {
  unicornLauncherProvider.useTinfoilShielding(true);
}]);

Notice that the unicorn provider is injected into the config function. This injection is done by a provider injector which is different from the regular instance injector, in that it instantiates and wires (injects) all provider instances only.

During application bootstrap, before Angular goes off creating all services, it configures and instantiates all providers. We call this the configuration phase of the application life-cycle. During this phase, services aren't accessible because they haven't been created yet.

Once the configuration phase is over, interaction with providers is disallowed and the process of creating services starts. We call this part of the application life-cycle the run phase.

Constant Recipe

We've just learned how Angular splits the life-cycle into configuration phase and run phase and how you can provide configuration to your application via the config function. Since the config function runs in the configuration phase when no services are available, it doesn't have access even to simple value objects created via the Value recipe.

Since simple values, like URL prefixes, don't have dependencies or configuration, it's often handy to make them available in both the configuration and run phases. This is what the Constant recipe is for.

Let's say that our unicornLauncher service can stamp a unicorn with the planet name it's being launched from if this name was provided during the configuration phase. The planet name is application specific and is used also by various controllers during the runtime of the application. We can then define the planet name as a constant like this:

myApp.constant('planetName', 'Greasy Giant');

We could then configure the unicornLauncherProvider like this:

myApp.config(['unicornLauncherProvider', 'planetName', function(unicornLauncherProvider, planetName) {
  unicornLauncherProvider.useTinfoilShielding(true);
  unicornLauncherProvider.stampText(planetName);
}]);

And since Constant recipe makes the value also available at runtime just like the Value recipe, we can also use it in our controller and template:

myApp.controller('DemoController', ["clientId", "planetName", function DemoController(clientId, planetName) {
  this.clientId = clientId;
  this.planetName = planetName;
}]);
<html ng-app="myApp">
  <body ng-controller="DemoController as demo">
   Client ID: {{demo.clientId}}
   <br>
   Planet Name: {{demo.planetName}}
  </body>
</html>

Special Purpose Objects

Earlier we mentioned that we also have special purpose objects that are different from services. These objects extend the framework as plugins and therefore must implement interfaces specified by Angular. These interfaces are Controller, Directive, Filter and Animation.

The instructions for the injector to create these special objects (with the exception of the Controller objects) use the Factory recipe behind the scenes.

Let's take a look at how we would create a very simple component via the directive api that depends on the planetName constant we've just defined and displays the planet name, in our case: "Planet Name: Greasy Giant".

Since the directives are registered via the Factory recipe, we can use the same syntax as with factories.

myApp.directive('myPlanet', ['planetName', function myPlanetDirectiveFactory(planetName) {
  // directive definition object
  return {
    restrict: 'E',
    scope: {},
    link: function($scope, $element) { $element.text('Planet: ' + planetName); }
  }
}]);

We can then use the component like this:

<html ng-app="myApp">
  <body>
   <my-planet></my-planet>
  </body>
</html>

Using Factory recipes, you can also define Angular's filters and animations, but the controllers are a bit special. You create a controller as a custom type that declares its dependencies as arguments for its constructor function. This constructor is then registered with a module. Let's take a look at the DemoController, created in one of the early examples:

myApp.controller('DemoController', ['clientId', function DemoController(clientId) {
  this.clientId = clientId;
}]);

The DemoController is instantiated via its constructor, every time the app needs an instance of DemoController (in our simple app it's just once). So unlike services, controllers are not singletons. The constructor is called with all the requested services, in our case the clientId service.

Conclusion

To wrap it up, let's summarize the most important points:

  • The injector uses recipes to create two types of objects: services and special purpose objects
  • There are five recipe types that define how to create objects: Value, Factory, Service, Provider and Constant.
  • Factory and Service are the most commonly used recipes. The only difference between them is that the Service recipe works better for objects of a custom type, while the Factory can produce JavaScript primitives and functions.
  • The Provider recipe is the core recipe type and all the other ones are just syntactic sugar on it.
  • Provider is the most complex recipe type. You don't need it unless you are building a reusable piece of code that needs global configuration.
  • All special purpose objects except for the Controller are defined via Factory recipes.
Features / Recipe type Factory Service Value Constant Provider
can have dependencies yes yes no no yes
uses type friendly injection no yes yes* yes* no
object available in config phase no no no yes yes**
can create functions yes yes yes yes yes
can create primitives yes no yes yes yes

* at the cost of eager initialization by using new operator directly

** the service object is not available during the config phase, but the provider instance is (see the unicornLauncherProvider example above).

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Licensed under the Creative Commons Attribution License 4.0.
https://code.angularjs.org/1.6.5/docs/guide/providers