Keypress working in chrome but not firefox

Question

I am trying to prevent the user from entering anything but a number. It works in Chrome but not Firefox. I have gone through many solutions but no luck. I have used keydown, keypress, different events etc. Please help. If it helps this is all in a aspx file.

onkeypress="return myFunction(event);"

<script type="text/javascript">
    // Check if key press is a number
    function myFunction(evt) {
        var e = event || evt; // for trans-browser compatibility
        var charCode = e.which || e.keyCode;

        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
            alert("Enter Numbers Only");
            return false;
        }
        return true;
    }
</script>

Answer 1

I don't understand this line

var e = event || evt; // for trans-browser compatibility

Where do you get event? This was throwing error in Firefox.

I modified the code and tested it with Chrome and Firefox. First the HTML with input

<input onkeypress="return validate(event);" />

and the function

// Check if key press is a number
function validate(e) 
{ 
    var charCode = e.which || e.keyCode;

    if (charCode > 31 && (charCode < 48 || charCode > 57)) 
    {
        alert("Enter Numbers Only");
        return false;
    }
    return true;
}

http://jsfiddle.net/bskqpgvy/2/