Adaptive skill practice — find your level, get sharper.

Testing practice

Write tests that catch bugs and survive refactors

Master the testing discipline — unit vs integration vs end-to-end and the test pyramid, test doubles (mock/stub/fake/spy), TDD and the AAA / given-when-then structure, code coverage and its limits, flaky tests and how to fix them, and advanced techniques (property-based, mutation, contract testing). Cross-language and evergreen. Part of the Developer track.

• Meets you at your level — never too easy, never too hard.

• Adapts every question to how you’re doing.

• Tracks your level over time. Skill practice, not a test.

New here? Read the Testing guide

A free 18-minute primer — the mental model, the mistakes beginners make, and what to practise first.

Curriculum

240 concepts across 30 chapters, from foundations through advanced practice — each one a concept the adaptive question bank can teach and test.

A. What a test is, and how a suite is built

B. Matchers and equality

C. The expect API in depth

D. Setup, teardown and isolation

E. Asynchronous tests

F. Test doubles — the taxonomy

G. Mock functions and call inspection

H. Module mocking and dependency seams

I. Fake timers and controlling the clock

J. Snapshot testing

K. Runner configuration and environments

L. The CLI, filtering and watch mode

M. Coverage — the metrics and their limits

N. Vitest — the Vite-native runner

O. Testing Library — querying like a user

P. Async DOM testing and user interaction

Q. Playwright locators and auto-waiting

R. End-to-end assertions and fixtures

S. Flakiness — causes and cures

T. Test strategy — what to test, and where

U. TDD and test design

V. Property-based, generative and fuzz testing

W. Running tests in CI

X. Reusable fixtures

Y. Parameterized cases

Z. Systematic concurrency testing

AA. Symbolic execution

AB. Jest under native ESM

AC. Code transformation

AD. Mocking a class

View the full curriculum →

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