SOURCE

class C {
    constructor() {
        this.num = Math.random();
    }
    rand() {
        console.log("Random:" + this.num);
    }
}

var c1 = new C();
c1.rand();

C.prototype.rand = function () {
    console.log("Random: " + Math.random(this.num * 1000));
};


var c2 = new C();
c2.rand();
c1.rand();