OpenLaszlo

<canvas>
  <script>
    function fibonacci(n) {
      if (n <= 1) return n;
      var a = 0, b = 1;
      for (var i = 2; i <= n; i++) {
        var temp = a + b;
        a = b;
        b = temp;
      }
      return b;
    }
  </script>
  <window title="Fibonacci Calculator" width="300" height="200">
    <simplelayout axis="y" spacing="10"/>
    <edittext id="input" width="200" text="10"/>
    <button onclick="output.setAttribute('text', 'fib(' + input.text + ') = ' + fibonacci(Number(input.text)))">
      Calculate
    </button>
    <text id="output">Result appears here</text>
  </window>
</canvas>