how to set char limit in prompt box using javascript?

What do you mean by this, in more details?

If you're talking about a js popup, then I don't think it's possible. However modal dialogs like jQueryUI popups, which may serve a similar purpose for you can be controlled as you can use a <input maxlength="20" ... />

BTW - if you want further info, post to the js forum, not php.

I have called following javascript function onclick

<script type="text/javascript">
        function getValue(){
           var retVal = prompt("Why you want to apply this job? : ", "write here");
           alert("You have entered : " +  retVal );
        }
    </script>

now how to set char limit in prompt box?

This cannot be done with the prompt() function, you shoul look towards the jQueryUI or something like that.

Ok moving...