How to produce a callback function inside functions

Question

I want to create a callback in order to retrieve its data later.

foo.js

const bar = require('bar'); 
bar.fix(part,(err,fixed)=>{
   if(err)
     return//handle err
   if (!fixed)
     return//handle not fixed

   else handle fixed
}); 

bar.js

module.exports.fix =(part,(err,fixed)=>{
//how can I manipulate err and fixed here to take effect on foo.js
})