express can 't find helper function module Error: Cannot find module './helpers/isPrime'?

I have an express.js block I want to refactor to be used in other parts of my webapp for example, I have function isPrime on my router file. I extract this block of code and put it in

./helpers/isPrime
    module.exports = function isPrime(num) {
        for(let i = 2 ; i < num ;i++){
            if(num % i == 0){
                return false;
            }
        }
        return true;
     }

when I added in route file I have this error Error: Cannot find module './helpers/isPrime' Require stack:

• /Users/auser/Documents/SandBox/AE_Technical_Challenge/routes/challengeRouter.js
• /Users/auser/Documents/SandBox/AE_Technical_Challenge/app.js

from express can 't find helper function module Error: Cannot find module './helpers/isPrime'?