improved fibonacci sequence (ing)

<script type="text/javascript">
/* This script and many more are available free online at
The JavaScript Source!! http://javascript.internet.com
Created by: Ben Clarke | http://benclarke.ca/ */

var i=0;
var fibs = []; // lookup table so it doesn't take forever
       
function fib(a) {
    var x, y, z;
    switch(a) {
        case 0: case 1: x = a; break;
        default:
            if(!fibs[a - 1]) {
                fibs[a - 1] = fib(a - 1);
            }
            if(!fibs[a - 2]) {
                fibs[a - 2] = fib(a - 2);
            }
           
            y = fibs[a - 1];
            z = fibs[a - 2];
            x = y + z;
    }
    return x;
}

function doFib() {
   
    var n = parseInt(document.getElementById("n").value);
    if(isNaN(n) || n < 1) {
        alert("Please enter a positive number for computations");
        return;
    }
   
    var a;
    var r = document.getElementById("r");
    r.value = "";
   
    for(var i=0; i<n; i++) {
        a = fib(i);
        r.value += " " + a;
    }
}
</script>


<p>
Computations: <input type="text" id="n"/></p>
<p>
<input type="text" id="r" style="width: 500px;"/></p>
<p>
<input type="button" onclick="doFib();" value="Calculate"/></p>