make textfield to accept numbers only

[Jagdeep Sharma]

Guys,

Please look at http://dev.sencha.com/deploy/dev/examples/grid/property-grid.html
if you try to edit border width and type alphabet 'a'. It won't accept. alphabet 'a' is not even visible for a second. If you type a number, it goes well. Now i want to achieve the same thing using javascript (using event handlers). How do i go about that.