How to get watchExpression in the listener of $watch function?

Question

Let's say you want to watch a model on scope which is defined as:

$scope.model = {firstName: 'John', lastName: 'Doe'};

the watcher can be written in 3 different ways below:

1.value

$scope.$watch('model', function(value) {
    //watches any change to each item
    //everytime the watcher will fire when firstName or lastName changes
    console.log(value.firstName); 
    console.log(value.lastName);
});

2.new and old value

$scope.$watch('model', function(newVal, oldVal) {
    //You get reference of new and old value of model
    //normally object reference equality is checked
    //if (newVal === oldVal)
    console.log(newVal.firstName); 
    console.log(oldVal.firstName);
});

3.directly from scope

$scope.$watch('model', function() {
    //watches any change to each item
    //access from scope directly
    //You are inside only when anything has changed on model
    console.log($scope.model.firstName); 
});

As an add-on you can also deep watch an object if necessary as:

$scope.$watch('model', function(newVal, oldVal) {
   //watch changes
}, true);

deep watch/object equality will trace changes from the object graph till the root node. For example for a model like:

$scope.model = { firstName: 'John', lastName: 'Doe', address: { zip: 1234 } };

watch would also fire for changes to zip in address.

It is not only the object you can watch but also specify a function whose return value can be watched. The function takes the scope as parameter by default:

$scope.$watch(function(scope) {
    return scope.model.firstName;
}, function(newVal, oldVal) {
    //Only watch the first name
    console.log(newVal.firstName); 
    console.log(oldVal.firstName);
});