What causes Flaky Tests, and how do you make tests deterministic (Determinism)?

Short answer

They may rely on unpredictable timing (random Timeouts), test order, the system clock, or leftover data; freeze time and clean the environment to address these causes.

Explanation

A flaky test sometimes passes and sometimes fails against the same code. To eliminate flakiness: 1) do not use setTimeout to wait for asynchronous events; listen for the actual events, 2) use timer-mocking libraries to control new Date(), and 3) isolate databases and generate separate IDs and data for each test.

Code example

// Use native mock timers in node:test
test.mock.timers.enable();
test.mock.timers.tick(5000);

Common mistakes

• Adding setTimeout(..., 2000) on the assumption that an operation will always finish in two seconds, which fails on slow CI servers.

Follow-up questions

• How do you isolate external services in integration tests with libraries such as nock or MSW?

Sources

• Node.js docs — Test runner: Mocking timers