How to handle events using java script?

function KeyPressed()
{
document.onkeydown = keydown;
function keydown(evt)
{
if (!evt)
evt = event;
if(evt.keyCode == 46 evt.keyCode == 8)
{
var value = document.getElementById('<%= txtValue.ClientID %>').value;
var show =parseInt(value );
if(show < 250)
{
document.getElementById('<%= txtValue.ClientID %>').innerText = show + 1;
}
}
else if(evt.keyCode != 37 evt.keyCode != 38 evt.keyCode != 39 evt.keyCode != 40)
{
var value = document.getElementById('<%= txtValue.ClientID %>').value;
var show = value-1;
document.getElementById('<%= txtValue.ClientID %>').innerText = show;
}
alert(evt.keyCode);
//return false;
}
}