Prefix 0001a, 0002b, 0002c series to start of image file name

I have tried the below script

var randomstring = (Math.random() + 1).toString(36).substring(7) + n;

            postTitle = postTitle;
            var imageExtension = $('input#image_extension_'+itemId+':hidden').attr('value');
            var imageTitle = postTitle + "-" + randomstring + imageExtension;

This code is generating like keyword-afdad234xeras3ss.jpg which random characters+numbers in the end.

I have tried some other solution except random coding but its not working. Is there any solution to fix this code so that each image is saved as

0001a-keyword1.jpg 0002b-keyword2.jpg 0003c-keyword3.jpg