controllers are created using a factory function? - angularjs

Question

I am new to angularJS and going through angular docs.I came across this line controllers are created using a factory function
I tried to find what that means and found what is factory and service and providers but that does not fit here.
How controllers are created using a factory function?
Please explain what is the meaning of factory in this context.

Answer 1

The key quote you are missing from the previous section you are referring to is this:
"First, there is a new JavaScript file that contains a so-called "controller". More exactly, the file contains a constructor function that creates the actual controller instance. "
If this were an actual angular factory, it would make more sense. But, controllers are instances just like Angular factories, services, and providers.
A factory, is actually a Javascript design pattern, maybe reading here it will make more sense.
For the controller to work, the instance must exist for the two-way binding to be able to take place. So basically, an instance of the controller is created. The angular controller page explains it well with:
"When a Controller is attached to the DOM via the ng-controller directive, Angular will instantiate a new Controller object, using the specified Controller's constructor function. A new child scope will be available as an injectable parameter to the Controller's constructor function as $scope." Here's the link.
In the event of controllers though, you would most likely store items on the $scope and not 'this'. So they separate controllers from factories this way as they do not return an accessible instance of themselves and instead bind their properties to the view through $scope or 'this'.
TO BE CLEAR, I'm not saying that they are referring to Angular factories. I believe the reason for this phrasing is tied to the same wording for the service factory function:
"Application developers are free to define their own services by registering the service's name and service factory function, with an Angular module.
The service factory function generates the single object or function that represents the service to the rest of the application. The object or function returned by the service is injected into any component (controller, service, filter or directive) that specifies a dependency on the service."
They give this example:
var myModule = angular.module('myModule', []);
myModule.factory('serviceId', function() {
var shinyNewServiceInstance;
// factory function body that constructs shinyNewServiceInstance
return shinyNewServiceInstance;
});
So when you see them say created from a factory function, they are just saying using the following pattern:
.controller('PersonCtrl', [
'PersonFactory'
function(PersonFactory) {
this.name = 'Tom';
console.log(PersonFactory.name); //would print 'Tom'
}]);
.factory("PersonFactory", [
function () {
return {
name: 'Tom'
};
}]);
I hope that helps, or maybe someone could be more concise in my explanation and edit this description.

Answer 2

What this means is that you provide AngularJS with a function that it can execute as many times as it needs to produce instances of your controller. So to take the example from the page you linked to:
angular.module('invoice3', ['finance3'])
.controller('InvoiceController', ['currencyConverter', function(currencyConverter) {
this.qty = 1;
this.cost = 2;
this.inCurr = 'EUR';
this.currencies = currencyConverter.currencies;
this.total = function total(outCurr) {
return currencyConverter.convert(this.qty * this.cost, this.inCurr, outCurr);
};
this.pay = function pay() {
window.alert("Thanks!");
};
}]);
That function that starts on line 2 with function(currencyConverter) { is the factory function.
Whenever the page has a location that uses an InvoiceController, AngularJS will (essentially) do the following with that factory function, passing in any dependencies that it has:
var currencyConverter = ...; // obtain a currency converter from the factory
var theController = new thatFactoryFunction(currencyConverter);
and then it will use the value that is returned as your controller. It will do this separately for each InvoiceController indicagted the page, creating a separate instance for each one.
(I stress that the code above is purely an illustration of what AngularJS is doing and not an actual representation of the code that it uses.)

Answer 3

The creation of a controller instance is interesting. One would expect that it is created with new InvoiceController(...), and it's also suggested by the sentence More exactly, the file contains a constructor function that creates the actual controller instance, but that's not the case. Actually it's created like this:
instance = Object.create(controllerPrototype);
and later the constructor function is called as function:
return fn.apply(self, args); //self == instance
To be honest, we can only guess what the author meant by factory function. It could be the fact that controllers are not created by new. Maybe the the constructor function is therefore referred to as factory or it's the internal factory function. It could also just be bad wording or even a mistake.