calculator2.js

function Calculator() {
	var methods = {
		"-": function(a, b) {
			return a - b;
		},
		"+": function(a,b) {
			return a + b;
		}
	};
	this.calculate = function(str) {
		var split = str.split(" "),
		a = +split[0],
		op = split[1],
		b = +split[2]

		if (!methods[op] || isNaN(a) || isNaN(b)) {
			return NaN;
		}
		return methods[op](a,b);
	}
	this.addMethod = function(name, func) {
		methods[name] = func;
	};
}

var calc = new Calculator;

calc.addMethod("*", function(a,b) {
	return a * b;
});
calc.addMethod("/", function(a,b) {
	return a / b;
});
calc.addMethod("**", function(a,b) {
	return Math.pow(a, b);
});
var result = calc.calculate("2 ** 3");
console.log(result);