Node.js Quality Control & Assurance 5

Question 1

What problem does `jest.useFakeTimers()` solve in unit tests?

Accepted Answer

It replaces `setTimeout`, `setInterval`, and `Date` with controllable fakes so time-dependent code can be tested synchronously

Answer

Fake timers let you advance time with `jest.advanceTimersByTime()` or `jest.runAllTimers()` without waiting for real clock ticks.

Question 2

What does the `--bail` option do when running Jest?

Accepted Answer

Stops the test run immediately after the first test suite failure

Answer

`--bail` (or `-b`) causes Jest to exit as soon as any test suite fails, useful in CI to get fast feedback without running the full suite.

Question 3

What is property-based testing, and which library implements it for Node.js?

Accepted Answer

Generating random inputs to find edge cases automatically; `fast-check` is a popular library

Answer

`fast-check` generates hundreds of random inputs per test run, attempting to falsify a property (invariant) that should always hold.

Question 4

In Node.js, what is the role of `--require` when running Mocha?

Accepted Answer

It loads a module before test files are executed, used for setup like registering Babel or loading env vars

Answer

`mocha --require @babel/register` is a common pattern that transpiles ES modules before tests run without changing test files.

Question 5

What is the primary goal of mutation testing tools like Stryker in a Node.js project?

Accepted Answer

To evaluate how well the test suite detects bugs by introducing small code changes (mutations) and checking if tests fail

Answer

Stryker generates mutants (e.g., changing `>` to `>=`) and reports a mutation score based on how many mutants the test suite catches.

Node.js Practice Test