[JavaScript] Updating the existing function behaviour

Ever find a situation when you want to change the function behaviour without duplicating the existing function code. Then you must be looking for this example.

Our test data object

var user = {
    name: 'testWorker',
    isOnLeave: true
};

Original function behaviour

var startWork = function(user){
    console.log('working...');    
}

[run]   ->startWork(user);
[output]->working...

Now if we want to change the function behaviour based on our data object.

startWork = (function(){
    //backup original function in private scope
    var _originalFn = startWork;
    //assign new behaviour
    return function(user){
        if(user.isOnLeave){
            console.log('on leave...');
        } else {
            _originalFn(user);
        }
    }
})();


[run]   ->startWork(user);
[output]->on leave...

//change data object
user.isOnLeave = false;
[run]   ->startWork(user);
[output]->working...

Note: It is not suggested to update the existing function behaviour. Because it can introduce defect which are very hard to debug and resolve.