How to prevent number input on keydown?

Use the keypress event instead. It's the only key event which will give you information about the character that was typed, via the which property in most browsers and (confusingly) the keyCode property in IE. Using that, you can conditionally suppress the keypress event based on the character typed. However, this will not help you prevent the user from pasting or dragging in text containing numeric characters, so you will still need some kind of extra validation.

My favourite reference for JavaScript key events: http://unixpapa.com/js/key.html

textBox.onkeypress = function(e) {
    e = e || window.event;
    var charCode = (typeof e.which == "undefined") ? e.keyCode : e.which;
    var charStr = String.fromCharCode(charCode);
    if (/\d/.test(charStr)) {
        return false;
    }
};

Try this to replace integer values:

<input onkeydown="Check(this);" onkeyup="Check(this);"/>

<script>
function Check(me) {
    me.value = me.value.replace(/[0-9]/g, "");
}
</script>

To prevent integer input:

<input onkeydown="Check(event);" onkeyup="Check(event);"/>

<script>
function Check(e) {
    var keyCode = (e.keyCode ? e.keyCode : e.which);
    if (keyCode > 47 && keyCode < 58) {
        e.preventDefault();
    }
}
</script>