let calculator={
    a: 0,
    b: 0,
    read(){
        this.a = prompt("please type a");
        this.b = prompt("please type b");
    },
    sum(){
        return Number(this.a) + Number(this.b);
    },
    mul(){
        return this.a * this.b;
    }
};

calculator.read();
alert(calculator.sum());
alert(calculator.mul());