Javascript prototypal inheritance - descendants override each other

I'm creating two objects (Inherits), both inherit from Base. The second object's property assignment overrides the value in the first object.

Any thoughts? How to make a proper inheritance so that Base class will contain common members for its inheritance descendants, but the descendants could assign values of their own without interfering each other.

var testParams1 = { title: "john" };
var testParams2 = { title: "mike" };

Function.prototype.inheritsFrom = function (baseClass)
{
    this.prototype = new baseClass;
    this.prototype.constructor = this;
    this.prototype.base = baseClass.prototype;
    return this;
};

function Base() {
    this.viewModel = {};
    this.viewModel.title = (function ()
    {
        var savedVal = "";
        return function (newVal)
        {
            if (typeof (newVal) === "undefined")
            {
                return savedVal;
            }
            savedVal = newVal;
        };
    })();
}
function Inherits(params) {
    this.viewModel.title(params.title);
}

Inherits.inheritsFrom(Base);

///// preparing code:
var testObj1 = new Inherits(testParams1);
var testObj2 = new Inherits(testParams2);

///// testing code:
equals(testObj1.viewModel.title(), testParams1.title, "first instance ok"); // returns "john"
equals(testObj2.viewModel.title(), testParams2.title, "second instance ok");  // also returns "john"