Using Jest without JavaScript modules

Needing to test a legacy application in which I couldn't modify the JavaScript files, I have found a way to test the scripts by attaching them to the jsdom installation.

Getting Started

In order to make this example work you just need to install Jest by typing

npm install

in your command line from the root of the project.

To run the tests you have to type

npm run test

or

npm run test_watch

if you want to keep watching your folders for changes.

Explanation

If you want to test a file that is not a module, in your test file you need to manually create a script object containing the code and to attach it to the window. In order to make this process easier I have created the loadExternalScripts function which is defined and exported in the loadExternalScripts.js file.

You can import it like this:

const loadExternalScripts = require('../jest-helpers/loadExternalScripts');

In order to use this function you neeed to extract the content of a script using node and the fs and path libraries, like this:

const srcFile = fs.readFileSync(path.resolve(__dirname, '../src/lib.js'), { encoding: 'utf-8' });

Inside your test file you can load those scripts in the beforeAll function:

beforeAll(() => {
    loadExternalScripts([jQueryFile, srcFile]);
});

At this time you can start testing the functions that you have defined inside your scripts.

If you start editing lib.test.js inside the __tests__ directory you can have an idea of how to use this configuration.