If one of the predefined cell formatters (integer, link, email, number, currency, checkbox) is not suitable in your scenario, you can use custom formatters.

With custom formatters you can define your own javascript client-side format and unformat functions. Format function are used to display the cell and unformat is used to unformat the cell in display mode and show it in edit mode.
Note the using of the special tag "js:"  in PHP code before the definition of the formatter function.
This tell the grid instance that this is a JS function which should be executed.
In the setJSCode you can write valid JS code. The code is putted at end of
all grid executions.