ReferenceError JWTAPIFunc is not defined

So I've been tasked with testing an express app that contains an AWS Lambda. I'm trying to test two methods in a file that look like this.

Validator.js

const Verifier = require('@companyName/api-jwt-validation');

JWTAPIFunc = function(callType){
  return function (req,res,next){
    .
    .
    .
    try {
    .
    .
    .
    } catch (authorizationError) {
        .
        .
        .
    }
  }
}

module.exports = {
      JWTAPIFunc:JWTAPIFunc
};

I want to test this JWTAPIFunc using jest so I have a test file that looks like this

Validator.test.js

const Verifier = require('@companyName/api-jwt-validation');
const Validator = require('../Validator')
jest.mock(Verifier);


test('Do something', ()=>{

});

When I run this empty test I get this error

● Test suite failed to run

ReferenceError: JWTAPIFunc is not defined

  1 | const Verifier = require('@companyName/api-jwt-validation');
  2 | 
> 3 |  JWTAPIFunc = function(callType){
    |                 ^
  4 |     return function (req,res,next){


  at Object.<anonymous> (src/Validator.js:3:17)
  at Object.<anonymous> (src/__tests__/Validator.test.js:3:1)

Why is it saying that this function is not defined? Sorry again if this is a stupid question.