Jest Tests, Now on CodePen

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Jest Tests, Now on CodePen
CodePen now supports Jest , the "delightful JavaScript testing framework".
Adding the Jest Block manually will scaffold a sample.test.js file for you, or adding any *.test.js file will automatically add the Block. Here's a basic unit test:
function add(a, b) {
return a + b;
}
describe('add', () => {
it('adds two numbers', () => {
expect(add(2, 2)).toBe(4);
});
});
Should the test fail, you'll get a warning diagnostic (but it won't otherwise fail to build your Pen).
You can also test UI by setting the enviornment to jsdom:
{
"testEnvironment": "jsdom"
}
This allows you to do things like mount components and test that they work how you think they will work, or even test your own CSS .
A visual representation of your passing/failing tests will automatically be created at /test-results.html which you can visit in the preview, or change where you'd like that file to be created:
{
"globals": {
"outputFilepath": "/index.html"
},
"testTimeout": 5000
}
Explore Jest Examples
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16 September 2026 · 16:00 UTC