https://learn.javascript.ru/constructor-new >> Задачи >> Создайте калькулятор | fun

calcTask.js

---
version 1
---

//Oops... well, there probably was so much 'shame' that I decided to delete it :)

---
version 2
---

function Calculator() {
    this.calculate = function (str) {
        var x = +str.split(' ')[0];
        var y = +str.split(' ')[2];
        var operator = str.split(' ')[1];
        if (operator === '+') return x + y;    
        if (operator === '-') return x - y;
        return 'this feature isn\'t been implemented yet';
    };
    this.addMethod = function (name, funArg, str) {
        var x = +str.split(' ')[0];
        var y = +str.split(' ')[2];
        var operator = str.split(' ')[1];
        if (name === operator) return funArg(x,y); // day two; yet wrong
    };
}

var calc = new Calculator;
calc.calculate('3 + 11');
calc.addMethod('*', (a,b) => a * b, '24 * 10');

---
version 3 
---
function Calculator() {
    this.container = {
        '+': function(a,b) { 
            return a + b; 
        },
        '-': (a,b) => a - b
    };
    this.calculate = function (str) {
        var x = +str.split(' ')[0];    // Number(str.split(' ')[0]);
        var y = +str.split(' ')[2];    
        var operator = str.split(' ')[1];
        if (operator in this.container) { // this.container.hasOwnProperty(operator);
            return this.container[operator](x,y);
        }
        return 'this functionality isn\'t supported... yet';
    };
    this.addMethod = function (name, funArg) {
        this.container[name] = funArg;
        return 'this feature is added :-) or has been already added earlier!';
    };
}

var calc = new Calculator;
calc.calculate('3 + 11');
calc.addMethod('*',(a,b) => a * b);
calc.calculate('2 * 4');