How to Unit-test A Private (non-exported) Function in JavaScript

How to Unit-test A Private (non-exported) Function in JavaScript

Originally published at on November 19, 2020.

Originally published at on November 19, 2020.

When writing unit-tests for JavaScript modules, we often encounter a dilemma wherein the module has some private functions that have not been exported. Testing a function that has been exported is easy since it can be imported in the unit testing framework, and the functionality can be tested. But how to unit-test a private (non-exported) function?

Testing exported function

Before we get to the private functions, let us get familiar with how we would test an exported function in JavaScript. We will be using Jest for writing this unit test. Let us assume a function foo such that:

// foo.js
export function foo() {
  return 'bar';

We will then write the corresponding unit test case for testing the functionality of the exported function as:

// foo.test.js
import { foo } from './foo.js'

describe('testing foo', () => {

Now that we are familiar with exported functions let us move to the next part. Let us first see what the issue would be first.

Non-exported (private) function

A non-exported function would be something like the secret function in this file:

// foo.js
let secret = () => '🤫'
export function foo() {
  return secret();

Now, if we were to test for baz in our unit test,

// foo.test.js
import secret from './foo.js'

// secret is undefined


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