How to import other js files in casperjs

Basically,I have extracted the re-usable functions into a file 'utility.js'.I want to import this file into each test script.I'm following the documentation here

When i try the above mentioned approach,I'm getting the following error.

'undefined' is not a function

Folder structure is like this

utility.js
test1.js
test2.js

Test file has following code as suggested by the above documentation

var require = patchRequire(require);
var helper = require('utility');

and I access the function in utility.js as follows helper.fn_do_something()

I tried to use other methods provided by casperJS,like

casper.options.clientscript = [relativepath];

But that didn't help.I'm getting the same error.

I'm using PhantomJS 1.9.8.