scope in javascript object

Your constructor function seems to have a bug. You are reading from n before assigning it.

Perhaps this would be clearer:

function Test() { this.sum = this.calculate(); }

Then get rid of the n value altogether.

Test.prototype.calculate = function() { return 5; }

Here I try to explain a bit.

function Test() {
    this.sum = n; // assign undefined to this.sum

    this.calculate(); // n = 5, but doesn't affect this.sum as undefined is already passed to sum
}

correct behavior( what you wanted)

function Test() {

    this.calculate(); 
    this.sum = n; 

}

Not sure what you are trying to do but try this:

var Test = (function () {
    function Test() {
        this.sum = this.calculate();
    }

    Test.prototype.calculate = function() {
        var n = 5;
        return n;
    }
    return Test;
})();

var mytest = new Test();
alert(mytest.sum); // 5

To answer your question - n was undefined because it had no value when you were trying to do this.sum = n;. It could have worked if you first invoked this.calculate() and then tried to assign this.sum = n;. But even in this case this was very wrong, since you were leaking variable n to the global namespace (when you do not explicitly initialize variable with var, it leaks to global namespace - window). So to illustrate what I mean - this could work:

var Test = (function () {
    function Test() {
        this.calculate();

        this.sum = n; // n is global now, hence accessible anywhere and is defined by this moment
    }

    Test.prototype.calculate = function() {
        n = 5; // not initialized with var so it leaks to global scope - gets accessible through window.n
        return n; // has no sense, since you do not use returned value anywhere
    }
    return Test;
})();

var mytest = new Test();