Document onkeydown get keyCode in javascript

document.onkeydown = myKeyDownHandler;
function myKeyDownHandler(event){
      alert(event.keyCode)
}

Input field enter integer only

Here's an example that limits a form field to contain only numeric input (on the fly, of course!).

<script type="text/javascript">
function numbersonly(e){
var unicode=e.charCode? e.charCode : e.keyCode
if (unicode!=8){ //if the key isn't the backspace key (which we should allow)
if (unicode<48||unicode>57) //if not a number
return false //disable key press
}
}
</script>

<form>
<input type="text" size=18 onkeypress="return numbersonly(event)">
</form>