How can I Test Output of a File using Mocha

I am currently writing a short piece of code for a Zapier integration. The code simply takes an input (provided by Zapier), and generates an output.

I am developing this locally because it is quicker and faster, but I also want to add tests to check that an input produces the right output.

I need to figure out how to require the file into the Mocha test, and then check the output is what it should be:

//index.js

// This is mock data which will be inputted automatically by Zapier
const inputData = {
  "data": "My String",
}
// ALL ABOVE THIS LINE IS REMOVED WHEN I COPY OVER TO ZAPIER



let output = {};

let newData = inputData.data;
newData = newData.toLowerCase();
output = {...output, newData};

console.log(output);

// test.js

const assert = require('assert');
const indexFile = require('../index');

describe('Output', () => {
  it('should return', () => {
     // todo test that string was converted to lower case correctly
  })
})

The only thing I can think of, is wrapping the whole file in a function, and adding a return statement, which may be the only way..?