How to test service files using wct in polymer if it doesn't contain any custom component

I would like to write unit test case for a service file named session.js, where there is no custom element is defined. Can we call functions defined in service file from Polymer WCT.

export const ValidateSession=()=>{  
       try{   
           return (logindate <= session);   
       }
       catch(e){   
           console.log(e.message);   
       }   
   }

In my service file, function is defined like above. How to write unit test case to check above function is being called or not.

Do everything you would do to test an element, but without an element fixture, or perhaps using a JS-only test suite:

suite('AwesomeLib', function() {
  test('is awesome', function() {
    assert.isTrue(AwesomeLib.awesome);
  });
});

Such test suites are described in the WCT README.