Angular service or factory?

tl;dr is at the end

In various AngularJS tutorials and documentation, the authors choose to use service or factory but don't explain why you would use one or the other. Few mention that value and constant are also options.

Let's see why you would use one over the other. We should also understand how providers work:

provider

Here's the source for the provider method:

function provider(name, provider_) {
    if (isFunction(provider_) || isArray(provider_)) {
        provider_ = providerInjector.instantiate(provider_);
    }
    if (!provider_.$get) {
        throw Error('Provider ' + name + ' must define $get factory method.');
    }
    return providerCache[name + providerSuffix] = provider_;
}

name is a string. provider_ can be one of three things:

1. function

   If a function is passed in, the function is called with dependency injection and should return an object with a $get method.

2. array

   An array will be treated like a function using Inline Annotation. It must also return an object with a $get method.

3. object

   If an object is passed in, it is simply expected to have a $get method.

Whatever the second arg to provider is, you eventually end up with an object that has a $get method. Here's an example showing what happens:

// You can run this

// Create a module
var hippo = angular.module('hippo', []);

// Register an object provider
hippo.provider('awesome', {
    $get: function() {
        return 'awesome data';
    }
});

// Get the injector (this happens behind the scenes in angular apps)
var injector = angular.injector(['hippo', 'ng']);

// Call a function with dependency injection
injector.invoke(function(awesome) {
    console.log('awesome == ' + awesome);
});

Once you understand providers you will see that factory, service, value and constant are just convenience methods for making providers.

factory

Here's the source:

function factory(name, factoryFn) {
    return provider(name, { $get: factoryFn });
}

So it lets you shorten the awesome provider creation code to this:

hippo.factory('awesome', function() {
    return 'awesome data';
})

service

Here's the source:

function service(name, constructor) {
    return factory(name, ['$injector', function($injector) {
        return $injector.instantiate(constructor);
    }]);
}

So it lets you make a factory that will instantiate a "class". For example:

var gandalf = angular.module('gandalf', []);

function Gandalf() {
    this.color = 'grey';
}
Gandalf.prototype.comeBack = function() {
    this.color = 'white';
}

gandalf.service('gandalfService', Gandalf);

var injector = angular.injector(['gandalf', 'ng']);

injector.invoke(function(gandalfService) {
    console.log(gandalfService.color);
    gandalfService.comeBack()
    console.log(gandalfService.color);
});

The above code will instantiate Gandalf, but remember that everything that uses the service will get the same instance! (which is a good thing).

value

Here's the source:

function value(name, value) {
    return factory(name, valueFn(value));
}

Using value would let you shorten the awesome provider to:

hippo.value('awesome', 'awesome data');

constant

Here's the source

function constant(name, value) {
    providerCache[name] = value;
    instanceCache[name] = value;
}

constant differs from value in that it's accessible during config. Here's how you use it:

var joe = angular.module('joe', []);

joe.constant('bobTheConstant', 'a value');
joe.value('samTheValue', 'a different value');

joe.config(function(bobTheConstant) {
    console.log(bobTheConstant);
});

joe.config(function(samTheValue) {
    console.log(samTheValue);
});

// This will fail with "Error: Unknown provider: samTheValue from joe"
var injector = angular.injector(['joe', 'ng']);

Read Module Loading & Dependencies in the Modules doc for more information on usage.

In summary

If you want your function to be called like a normal function, use factory. If you want your function to be instantiated with the new operator, use service. If you don't know the difference, use factory.

This is the (great) documentation for each function in the AngularJS source:

1. factory

   A short hand for configuring services if only `$get` method is required.

2. service

   A short hand for registering service of given class.

3. value

   A short hand for configuring services if the `$get` method is a constant.

4. constant

   A constant value, but unlike {@link AUTO.$provide#value value} it can be injected into configuration function (other modules) and it is not interceptable by {@link AUTO.$provide#decorator decorator}.