Function calling within objects in Javascript

Question

I have a object in javascript and I am going to call its weeklyTargetConstraint(). Problem is that it will call a method displayWeeklyTargetPrompt(), but this will not carry the object reference in the method call.

Why is the console printing out the Window object? And how am a suppose to do this properly?

function main() {
     c = new Constraints(5,6);
     c.weeklyTargetConstraint();  
}


function Constraints(weekNum, dayID) {
     //setup
}    

Constraints.prototype = {
    weeklyTargetConstraint: function () {
        var state = $("#hfWeeklyFillOverride", weekNumber).val();
        this.constraintQty(this.fillTarget, state, this.displayWeeklyTargetPrompt);
    ///this will call displayWeeklyTargetPrompt()...
    },
    displayWeeklyTargetPrompt: function () {
        console.log(this);               // this is the Window Object
        console.log(this.weekNumber);    // its a input with id = weekNumber
        loadWeeklyFillTargetMsg(this.weekNumber);
        $('#weeklyTargetConfirm').dialog('open');
    },
// other stuff here
}