AngularJS has a concept of powerful data binding. It accomplishes this by keeping a watch on its scope variables. When we change the the variable on $scope object, the $scope object gets automatically changed and vice-versa. Most of the time angular calls the $watch function, but in some cases to  update the new values,  you may need to call it by yourself.

There are two ways to set up a $watch-

1.by expression- listens(or watches) the $scope and converts to function.
2.by function- it simply watches the function and no conversion is done.

Using $watch:-

$watch function has three parameters- expression,listener and equality object. The last two parameters are optional.
1.

$scope.$watch(test, function(newVal, oldVal) {
    console.log(newVal, oldVal);
});

It will watch ‘test’ which is an expression against $scope.

2.    

$scope.$watch(function() {
            return $scope.test;
        }, function(newVal, oldVal) {
            console.log(newVal, oldVal);
        });

Both are same in functionality. All watchers are evaluated when $digest is called.