Fiddlin

fiddle.css

@import url('http://twitter.github.com/bootstrap/assets/css/bootstrap.css');

fiddle.html

<div class="container">
    <input type="text" name="Calculate" id="expr" value="" />
    <a class="btn btn-primary" id="calc" href=""> Calculate </a>
    <input type="text" name="Result" id="res" value="" />
</div>

fiddle.js

var Stack = function Stack(size) {
    this.size = size;
    this.stack = new Array(size);
    this.head = 0;
}

Stack.prototype.push =
function(element) {
    if (this.head > this.size) {
        return false;
    } else  {
        this.stack[this.head++] = element;
        return true;
    }
};

Stack.prototype.pop =
function() {
    if (this.this.head < 0) {
        return null;
    } else {
        return this.stack[this.head--];
    }
};

Stack.prototype.peek =
function() {
    return this.stack[this.head];
};

var Calculator = function Calculator() {
    this.stack = new Stack(128)
}

Calculator.prototype.calculate =
function(expression) {
    var s = this.stack,
        cur = "";

    for (var i = 0; i < expression.length; i += 1) {
        var c = expression[i];

        if (/[0-9]/.test(c)) {
            cur += c;
        } else if (c === " ") {
            s.push(cur | 0);
            cur = "";
        } else if (/\*\-\/\+/.test(c)) {
            var op1 = s.pop(),
                op2 = s.pop();
            switch (c) {
                case '*':
                    s.push(op1 * op2);
                    break;
                case '/':
                    s.push(op1 / op2);
                    break;
                case '+':
                    s.push(op1 + op2);
                    break;
                case '-':
                    s.push(op1 - op2);
                    break;
            }
        } else {
            return null
        }
    }

    return s.pop()
};

window.calculator = new Calculator();
window.addEvent('domready', function () {
    var expr = $("expr"), 
        res = $("res"),
        calc = $("calc");    
    calc.addEvent('click', function () {
        res.set('value', calculator.calculate(expr.value));
    });
});

fiddle.manifest

name: DC
description: Desk calculator
authors:
    - Adam Rudd
resources
    - http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/2.1.0/bootstrap.min.js
normalize_css: no