calc.js

// Simple arithemetic parser.  
// Follows order of operations, only supports positive integers
// Only operators are +, -, *, and /.  Does not support grouping with ()
var calc = (function () {
	var calc,
		sum,
		negasum,
		product,
		dividend;
		
	sum = function(args) {
		var r = calc(args[0]),
			i,
			l;
		
		for (i = 1, l = args.length; i < l; i += 1) {
			r += calc(args[i]);
		}
		
		return r;
	};
	
	negasum = function(args) {
		var r = calc(args[0]),
			i,
			l;
		
		for (i = 1, l = args.length; i < l; i += 1) {
			r -= calc(args[i]);
		}
		
		return r;
	};
	
	product = function(args) {
		var r = calc(args[0]),
			i,
			l;
		
		for (i = 1, l = args.length; i < l; i += 1) {
			r *= calc(args[i]);
		}
		
		return r;
	};
	
	dividend = function(args) {
		var r = calc(args[0]),
			i,
			l;
		
		for (i = 1, l = args.length; i < l; i += 1) {
			r /= calc(args[i]);
		}
		
		return r;
	};
	
	calc = function(input) {
		if (input.indexOf("+") >= 0) {
			return sum(input.split("+"));
		} else if (input.indexOf("-") >= 0) {
			return negasum(input.split("-"));
		} else if (input.indexOf("*") >= 0) {
			return product(input.split("*"));
		} else if (input.indexOf("/") >= 0) {
			return dividend(input.split("/"));
		} else {
			return parseInt(input, 10);
		}
	};
	
	return calc;
}());

// Robust test suite, benchmarked against native eval()
var string = "1+1*2+5-10/2";
console.log("calc(%s) -> %o", string, calc(string));
console.log("eval(%s) -> %o", string, eval(string));