function calling a service only works when initializing - angularjs

Question

Getting started with AngularJS working through the W3schools tutorial.
I'm trying to expand on some of the sample code by creating an input which is used by the function getHex(), which uses the service hexafy.
<div ng-app="myApp" ng-controller="myCtrl">
<p>The hexadecimal value of <input ng-model="inputNo"/> is:</p>
<h1>{{getHex()}}</h1>
</div>
<p>A custom service with a method that converts a given number into a hexadecimal number.</p>
<script>
var app = angular.module('myApp', []);
app.service('hexafy', function() {
this.myFunc = function (x) {
return x.toString(16);
}
});
app.controller('myCtrl', function($scope, hexafy) {
$scope.inputNo = 255;
$scope.getHex = function() {
return hexafy.myFunc($scope.inputNo);
};
});
</script>
When the page firsts load, the input contains 255 and the {{getHex()}} is rendered as ff, so the function appears to run correctly.
But when I change the value in the input, it just returns exactly my input into {{getHex()}} without hexify-ing it. getHex() is supposed to return the result of hexafy.myFunc.

Answer 1

convert the $scope.inputNo to int before sending to the service
return hexafy.myFunc(parseInt($scope.inputNo));
Demo

Answer 2

Convert the sent payload to a javascript Number before converting to hex.
var app = angular.module('myApp', []);
app.service('hexafy', function() {
var hexafyService = {};
var _myFunc = function(x) {
return Number(x).toString(16);
}
hexafyService.myFunc = _myFunc;
return hexafyService;
});
app.controller('myCtrl', function($scope, hexafy) {
$scope.inputNo = 200;
$scope.getHex = function() {
$scope.hexNo = hexafy.myFunc($scope.inputNo);
};
$scope.getHex();
});
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="myApp" ng-controller="myCtrl">
<p>The hexadecimal value of <input ng-model="inputNo" ng-change="getHex()" /> is:</p>
<h1>{{hexNo}}</h1>
</div>
<p>A custom service with a method that converts a given number into a hexadecimal number.</p>