Unit testing an inner function?

I have a function that has inner functions, for my unit test, I only want to test the functionality of the inner function, but when I export the function and call the inner function, npm tests returns an error.

In my main.js

mainFunction = () => {
 functionToBeTested = () => {
///some code
}
}
module.exports = {mainFunction: mainFunction}

In my test.js

const chai    = require("chai");
const assert  = require("chai").assert;
const mainFunction = require("./main");

describe ("test", () => {
 it("returns results", () => {
  let result = mainfunction.functionToBeTested(args);
  //equal code
  })
})

But when I run Npm Test, it says mainfunction.functionToBeTested is not a function. What am I doing wrong?