Streng manipulation.

Har selv lavet svaret.

<form name="myform"
<input type="Text" name="one">
<input type="Button" value=" test " onclick="fChar();">
</form>

<script language="JavaScript">
function fChar(){
var str = document.myform.one.value;
var s = str.charAt('0') + str.charAt('1') + str.charAt('2') + str.charAt('3');
    if (str != ""){

    while(s.indexOf("1") != -1)
    s = s.replace("1","a");

    while(s.indexOf("2") != -1)
    s = s.replace("2","b");
   
    alert(s);
    } else {
    alert('Boxen er tom !');
    }
}
</script>