How to use CodeceptJS to unit-test a JS function

I've set up CodeceptJS for a project and use it to test various end-to-end scenarios.

Now I want to extend the tests-suite to also run unit-tests to verify functionality of custom JS functions.

For example: I have a global object App that has a version attribute. As a first test, I want to confirm that App.version is present and has a value.

My first attempt is a test.js file with the following code:

Feature('Unit Tests');

Scenario('Test App presence', ({ I }) => {
    I.amOnPage('/');

    I.executeScript(function() {return App.version})
        .then(function(value) { I.say(value) } );
});

Problems with this code

1. The major issue: How can I assert that the App.version is present?
   My script can display the value but does not fail if it's missing
2. My code is very complex for such a simple test.
   I'm sure there's a cleaner/faster way to perform that test, right?

Here is a solution that works for me:

Read data from the browser:

1. I created a custom helper via npx codecept gh and named it BrowserAccess.
2. The helper function getBrowserData uses this.helpers['Puppeteer'].page.evaluate() to run and return custom code from the browser scope. Documentation for .evaluate()

Custom assertions:

1. Install the codeceptjs-assert package, e.g. npm i codeceptjs-assert
2. Add the AssertWrapper-helper to the codecept-config file. This enables checks like I.assert(a, b)

Full Code

codecept.conf.js

exports.config = {
    helpers: {
        AssertWrapper: {
            require: "codeceptjs-assert"
        },
        BrowserAccess: {
            require: './browseraccess_helper.js'
        },
        ...
    },
    ...
}

browseraccess_helper.js

const Helper = require('@codeceptjs/helper');

class BrowserAccess extends Helper {
    async getBrowserData(symbolName) {
        const currentPage = this.helpers['Puppeteer'].page;
        let res;

        try {
            res = await currentPage.evaluate((evalVar) => {
                let res;
                try {
                    res = eval(evalVar);
                } catch (e) {
                }
                return Promise.resolve(res);
            }, symbolName);
        } catch (err) {
            res = null;
        }

        return res;
    }
}

jsapp_test.js (the test is now async)

Feature('Unit Tests');

Scenario('Test App presence', async ({ I }) => {
    I.amOnPage('/');

    const version = await I.getBrowserData('App.version');
    I.assertOk(version);
});