arithmetic-evaluator.js

function Value(value){
	this.value = value || 0;
}

Value.prototype.eval = function(){ return this; };
Value.prototype.valueOf = function(){ return this.value.valueOf(); };
Value.prototype.toString = function(){ return this.value.toString(); };

var createOperator = function(){
	function reduce(oper){
		return new Value(oper.apply(null, this.expressions.map(function(expr){ return expr.eval();})));
	}
	function toString(name){
		return this.expressions.map(function(expr){ return expr.toString();}).join(' ' + name + ' ');
	}
	return function(name, oper){
		return function(){
			this.expressions = [].slice.call(arguments);
			this.eval = reduce.bind(this,oper);
			this.toString = toString.bind(this,name);
		};
	};
}();

var Add = createOperator("+", function(a,b){ return a + b;});
var Sub = createOperator("-", function(a,b){ return a - b;});
var Mul = createOperator("*", function(a,b){ return a * b;});
var Div = createOperator("/", function(a,b){ return a / b;});
var Exp = createOperator("^", function(a,b){ return Math.pow(a,b);});

describe("Kata Test Suite", function(){
  it("should evaluate expressions", function(){
    var exp = new Add(new Mul(new Value(3),new Value(5)),new Sub(new Mul(new Div(new Value(6),new Value(2)) ,new Exp(new Value(10),new Value(2))), new Value(273)));
    Test.assertEquals(exp.toString(), "3 * 5 + 6 / 2 * 10 ^ 2 - 273");
    Test.assertEquals(exp.eval().valueOf(), 42);
  });
});