How do I define constructor in service and factory in Angular?

Question

i am little bit familiar with angular. still on learning process. working with ng version 1.4.8. so i like to know how could i define constructor function in service and factory.

here is one sample service .now tell me how to define constructor in service or factory ?

angular.module('myApp').service('helloService',function($timeout){
  this.sayHello=function(name){
    $timeout(function(){
      alert('Hello '+name);
    },2000);
  }
});

angular.module('myApp').controller('TestController',
  function(helloService){
  helloService.sayHello('AngularJS'); // Alerts Hello AngularJS
});

Answer 1

The function you pass to .service gets called with new, thus it is already basically a constructor. It is a "constructor function" and it implicitly returns an object, which is the singleton:

angular.module('myApp').service('helloService',function($timeout){
  // This constructor function implicitly returns an object. It is called
  // only once by Angular to create a singleton.

  this.sayHello = function(name) {
    // etc
  }
});

Just to illustrate, if you passed an ES6 class to .service (which does have a constructor) instead of a constructor function, that constructor would be called when the singleton is created:

class HelloService {
    constructor($timeout) {
        // Called once when the singleton is created
    }

     sayHello(name) {
         // etc
     }
}

angular.module('myApp').service('helloService', HelloService);

Using .factory is similar, but it doesn't get called with new. So the function you use in this case has to return a singleton object explicitly:

angular.module('myApp').factory('helloService',function($timeout){
  // This factory function explicitly returns an object. It is called
  // only once by Angular to create a singleton.

  return {
      sayHello: function(name) {
          // etc
      }
  };
});

Edit: As mentioned by @Vladimir Zdenek, these "constructors" cannot be used to configure the singleton externally. However, I interpreted the question to mean "Where can I put code that runs when the singleton is created?". Singletons may need to initialize data, so that initialization can go in the "constructor".