class Calculator {
  constructor(value) {
    this.value = value;
  }
  
  add(num) {
    this.value += num;
    return this;
  }
  
  multiply(num) {
    this.value *= num;
    return this;
  }
}

const calc = new Calculator(5);
const addMethod = calc.add;
const result = addMethod.call(calc, 3).multiply(2);
console.log(result.value);