class Calc {
    constructor() {
        this.lastSum  = -1;
    }

    getSum(n1, n2) {
        this.lastSum = n1 + n2;
        return this.lastSum;
    }
}

let calc = new Calc();
let sum1 = calc.getSum(5, 3);
//sum1 is 8

let sum2 = calc.getSum(2, 3);
//sum2 is 5

console.log("sum1 = " + sum1);
console.log("sum2 = " + sum2);
console.log("calc.lastSum = " + calc.lastSum);