Javascript this points to Window object

Question

I have the following code. I expected to see "archive" object on my firebug console, but I see Window object. Is it normal?

var archive = function(){}

archive.prototype.action = {
    test: function(callback){
        callback();
    },
    test2: function(){
        console.log(this);
    }
}

var oArchive = new archive();
oArchive.action.test(oArchive.action.test2);

Answer 1

oArchive.action.test2 gets you a reference to a function that callback then points to, but that function is then called using callback(), which means it is not called as a method and hence this is the global object. The key point is that this is not bound to a function: it's determined by how the function is called.

In this case you could explicitly make this point to the action object (but not the archive object) by using the callback function's call or apply method:

test: function(callback) {
    callback.call(this);
},

To get it this to be the archive object instead, you'll need to pass the archive object in:

var archive = function(){}

archive.prototype.action = {
    test: function(callback, archive){
        callback.call(archive);
    },
    test2: function(){
        console.log(this);
    }
}

var oArchive = new archive();
oArchive.action.test(oArchive.action.test2, oArchive);