function Foo(name, surname, age) {
    this.name = name;
    this.surname = surname;
    this.age = age;
}

Foo.prototype = {
    constructor: Foo,
    x: 12,
    y: 13
};

var a = new Foo("Philipp", "Plotnikov", 17);
console.log(a.prototype.constructor.toString())