"undefined global variable:i" on javascript

I’m making a javascript function on blank application,
the script is

?>
<script type=“text/javascript”>
window.onload = fillDropDown();

function fillDropDown() {
var ddl = document.getElementById( ‘myDropdown’ );
var theOption = new Option;
var x;
var i;

for(i = 0; i < 999; i++) {
x = i + 1;
theOption.text = x;
theOption.value = x;
ddl.options[i] = theOption;
}
}
</script>

<form id=“myForm”>
<select id=“myDropdown”></select>
</form>
<?php

but when run the application i get the message:
undefined global variable:i

so why happen this and what i can do?
i need make this application.
thanks in advance