Function.prototype.bind()

gistfile1.js

function Shape() {
    this.width = 100;
    this.height = 100;
}

Shape.prototype.area = function() {
    return this.width * this.height;
}

var square = new Shape();
square.area(); // returns 10000

var rectangle = {
    width: 100,
    height: 50
};

var rectangleArea = Shape.prototype.area.bind( rectangle );
rectangleArea(); // returns 5000