Vados73,
<script>
function fn(number) {
var result = [];
while (number) {
result.push(number % 2);
number = Math.floor(number / 2);
}
result.reverse();
result = "0000" + result.join("");
result = result.slice(-4);
return result;
}
for (var i = 0; i < 16; i++) {
document.write(i + " => " + fn(i) + "<br>");
};
</script>