Http request in constructor doesn't change attribute value - angularjs

Question

I've got a problem with this constructor:
function ApiManager() {
this.api= new ApiInterface();
this.apiVersion = -1;
this.api.getVersion()
.then(function(version) {
console.log(version); // "1.0"
this.apiVersion = version;
console.log(this.apiVersion); // "1.0"
}, function(error) {
console.log("Couldn't find API version.")
});
}
ApiManager.prototype = {
getApiVersion: function() {
return this.apiVersion; // "-1"
}
};
I have an object with the attribute apiVersion, which is initialized with the value -1. After that an HTTP request is executed by the ApiInterface and assigns apiVersion the value 1.0. Later I call the function getApiVersion and it returns the old value -1.
I'm new to AngularJS and probably it's a silly rookie mistake, but I don't see what I've got wrong.

Answer 1

This is what the Promises/A+ Spec says about the this keyword inside .then functions:
That is, in strict mode this will be undefined inside of them; in sloppy mode, it will be the global object.
-- Promises/A+ Spec -- Note 3.2
So in your code, you need to explicitly bind instead of using the this keyword.
function ApiManager() {
this.api= new ApiInterface();
this.apiVersion = -1;
//bind 'this' keyword to a var
var vm = this;
this.api.getVersion()
.then(function(version) {
console.log(version); // "1.0"
//use that binding
vm.apiVersion = version;
console.log(vm.apiVersion); // "1.0"
}, function(error) {
console.log("Couldn't find API version.")
});
}

Answer 2

I also tried the version with the promise and this is the result:
function ApiManager() {
this.api= new ApiInterface();
this.apiVersion = api.getVersion();
}
ApiManager.prototype = {
getApiVersion: function() {
return this.apiVersion;
}
};
The ApiInterface returns a promise in the getVersion method.
getVersion: function() {
return $http({
method: 'GET',
url: myUrl;
}).then(function successCallback(response) {
return response.data.jsonapi.version;
}, function errorCallback(response) {
return $q.reject(response.data);
});
},
Later on, I use it this way in my controller:
$scope.printApiVersion = function () {
var promise = manager.getApiVersion();
promise.then(function(version) {
console.log(version);
}, function(error) {
console.log("Couldn't retrieve API version.")
});
}
This works great for me, but I don't know if it is considered good style.