Find the production issues that tests can miss.

Every level is a Node.js service that passes all its tests and still falls over. Reproduce the failure, fix the code in your browser, then find out why nobody caught it.

Play level 1See all levels
TypeScript or JavaScript / runs in your browser / nothing to install
checkout.jslevel 01
let stock = 1;

app.post('/buy', async () => {
  if (stock > 0) {
    await db.reserve();  // slow call
    stock--;
    return { ok: true };
  }
  return { ok: false };
});
STOCK
1
One item left. Three customers click buy together.

Fixing it is half the level.

When the hidden tests pass, you get the part other sites skip: why your tests never saw it.

  1. Read the incident

    An alert, some logs and a symptom. The visible test is already green.

  2. Fix the code

    Edit the service in your browser, replay the traffic, and submit. Hidden tests decide.

  3. Learn why it slipped through

    A short explanation of what production did that the tests did not, and the rule to remember.

Level 1 ยท The last headlamp
Solved
TIME04:12
ATTEMPTS2
HIDDEN TESTS3/3
WHY THE TESTS MISSED ITThe visible test sent one order at a time. In production three arrived together, and all of them passed the stock check before any updated it.
Example result