Undefined "this" in Angular factory - angularjs

Question

I have am Angular Factory that creates global on screen alerts, but unsetting the alerts errors with an
"TypeError: undefined is not a function" error.
My Factory is:
'use strict';
angular.module('meanstackStarterApp')
.factory('Alerts', function ($timeout) {
var alertType = null;
var alertTitle = null;
var alertMessage = null;
return {
setMessage: function(type, title, message, timed) {
console.log("in");
this.alertType = type;
this.alertTitle = title;
this.alertMessage = message;
if (type == "alert-danger")
{
alertMessage = message + alertMessages.alertMessage_GlobalTryAgain;
}
if (timed)
{
// Set Timeout on unset
$timeout(this.unsetMessage(),5000);
}
},
unsetMessage: function() {
console.log("out");
this.alertType = null;
this.alertTitle = null;
this.alertMessage = null;
}
};
});
The setMessage function works fine but the unsetMessage errors on the this.
If I remove this prefix from the variables, the code runs through without erroring, but does not actually set the factory variables.
What am I missing?

Answer 1

I have here something like you, I'm changing your code to look like mine:
angular.module('meanstackStarterApp')
.factory('Alerts', function ($timeout) {
var self = this;
var alertType = null;
var alertTitle = null;
var alertMessage = null;
self.setMessage = function(type, title, message, timed) {
console.log("in");
self.alertType = type;
self.alertTitle = title;
self.alertMessage = message;
if (type == "alert-danger")
{
alertMessage = message + alertMessages.alertMessage_GlobalTryAgain;
}
if (timed)
{
// Set Timeout on unset
$timeout(self.unsetMessage(this),5000);
}
};
self.unsetMessage = function() {
console.log("out");
self.alertType = null;
self.alertTitle = null;
self.alertMessage = null;
};
return self;
});