Very Simple Javascript Expression Evaluator

evaluate.js

function evaluate(input) {
    // Supported ops (brackets arent really there)
    var ops = ["(", ")", "^", "/", "*", "%", "+", "-"];
    // Check brackets
    var j = 0;
    for (var i = 0; i < input.length; i++) {
        if (input.charAt(i) == "(")
            j++;
        if (input.charAt(i) == ")")
            j--;
    }
    if (j != 0) {
        throw "Bracket Mismatch!";
    }
    // Iterate through terms
    var node = function(val) {
        return {
            element: val,
            left: undefined,
            right: undefined
        };
    };

    var opStack = []; // top of stack is .length - 1
    var valStack = []; // top of stack is .length - 1
    function push() {
        // each op has 2 parameters
        var n = opStack.pop();
        if (valStack.length < 2)
            throw "Lonely operators! Invalid expression!";
        n.right = valStack.pop(); // every op has 2 params
        n.left = valStack.pop();

        valStack.push(n); // push back onto value stack
    }
    for (var i = 0; i < input.length; i++) {
        var current = input.charAt(i);
        if (current == "(") {
            // Open
            opStack.push(node(current));
            continue;
        }
        if (current == ")") {
            // Close
            while (opStack[opStack.length - 1].element != "(") {
                push();
            }
            opStack.pop(); // remove the other bracket "("
            continue;
        }
        if (ops.indexOf(current) != -1) {
            // Op
            while (opStack.length > 0 && opStack[opStack.length - 1].element != "(" && ops.indexOf(current) >= ops.indexOf(opStack[opStack.length - 1])) {
                push();
            }
            opStack.push(node(current));
        } else {
            if (current != " ") {
				// Handle this number as anything till next op.
                for (var x = i + 1; x < input.length; x++) {
                    if (ops.indexOf(input.charAt(x)) == -1) {
                        i++;
                        current = current + input.charAt(x);
                    } else {
                        break;
                    }
                }
                // Number
                valStack.push(node(current));
            }
        }
    }
    // make remaining
    while (opStack.length > 0) {
        push();
    }

    // print
    function traverse(node) {
        if (ops.indexOf(node.element) != -1 && node.left != undefined && node.right != undefined) {
            var x = traverse(node.left);
            var y = traverse(node.right);
            var i = ops.indexOf(node.element);
            if (i == 2)
                return x ^ y;
            if (i == 3)
                return x / y;
            if (i == 4)
                return x * y;
            if (i == 5)
                return x % y;
            if (i == 6)
                return x + y;
            if (i == 7)
                return x - y;
            return 0;
        } else {
            return parseInt(node.element);
        }
    }
    return traverse(valStack[valStack.length - 1]);
}