onkeypress event in JS

I have below JS method which is called on 'onkeypress' event of a textbox. Problem is button name doesnt change for value 50001, it reflects only when i enter more than 5 digits. please help, how cna we do this in JS.
function checkvalue(e)  { 
var f=e.value;
if (f > 50000)   
 {    
alert(f);        
$('#btnSub').val("Submit to Manager");  
  } 
 else  
$('#btnSub').val("Submit");
 }