Can't find issue in simple js for-loop
<p>I am trying out javascript for (more-or-less) the first time and find myself completely baffled by the following .js script.</p>

<pre><code>var pair = newArray();
var id = newArray();
var pairs = 2;

function newGame(){

    var randomid = 0;

    alert("newGame() called!");

    // Sets a specific part of the image sprite to each pair[].
    for (var i=0 ;i &lt; pairs; i++){
        alert("For loop started!");
        pair[i] = "url(Cardfront.jpg) -"+100 * Math.floor((Math.random()*13)+0)+"px -"+200 * Math.floor((Math.random()*4)+0)+"px";
        // For every pair, assigns a part of the image sprite to two id[]-s.
        alert("Pair " + i + "is " + pair[i]);
        for(var j=0; j &lt; 2; j++) {
            //the range of possible id-s the total number of cards - double the amount of pairs.
            randomid = Math.floor((Math.random()*pairs*2)+0);
            if (id[randomid] === null){
                id[randomid] = pair[i];
                alert("ID " + randomid + "is " + id[randomid]);
            }
            else j--;
        }
    }

    alert("This is called after the for loop!");
}
</code></pre>

<p>When I call newGame() through a button, I receive the "newGame() called!" and then "This is called after the for loop!" alerts, then nothing.</p>

<p>I've spent a while googling and poking around trying to figure this out, but I'm at the end of my wits, it seems.</p>
