When you create a data binding from somewhere in your view to a variable on the $scope object, AngularJS creates a "watch" internally. A watch means that AngularJS watches changes in the variable on the $scope object. The framework is "watching" the variable. Watches are created using the $scope.$watch() function
<!DOCTYPE html>
<html lang="en" >
<head>
<title>$digest,$watch</title>
<script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js"></script>
<script type="text/javascript">
var application=angular.module('myapp',[]);
application.controller('mycontroller',function($scope)
{
$scope.myRandomNumber=Math.random();
$scope.counter=-1;
document.querySelector('input').addEventListener('click',function(){
$scope.myRandomNumber=Math.random();
$scope.$digest();
$scope.$watch('myRandomNumber',function(newValue,oldValue)
{
if($scope.counter==5)
alert('You reach at 5');
$scope.counter++;
}
);
},false);
}
);
</script>
</head>
<body ng-app="myapp" ng-controller="mycontroller">
{{myRandomNumber}}
<br/>
<input type="button" value="Generate Random Number"/>
</body>
</html>
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