Timothy Pirez answer was very correct but if you need to replace the numbers with commas Immediately as user types in textfield, u might want to use the Keyup function.
$('#textfield').live('keyup', function (event) {
var value=$('#textfield').val();
if(event.which >= 37 && event.which <= 40){
event.preventDefault();
}
var newvalue=value.replace(/,/g, '');
var valuewithcomma=Number(newvalue).toLocaleString('en');
$('#textfield').val(valuewithcomma);
});
<form><input type="text" id="textfield" ></form>