5. Test Running with Jest

Question 1

What are the rules determining whether a JS file is a test file for Jest?

Answer 1

1. Any JS file inside a folder named \_\_tests\_\_ are considered tests.
2. Any JS files with *.spec.js or *.test.js extension in their names are considered tests.

Question 2

What are the pros and cons of having tests in their own folders?

Answer 2

1. Easily distinguish between test and non-test files.
2. Unrelated files can share a folder (i.e., a loginService test and a cryptoHash test).
3. It’s very easy to isolate a particular set of tests that are in the same folder.
4. Tests can be named anything but must be inside an appropriately named folder (i.e. __tests__), to be recognised.

Question 3

What are the pros and cons of having tests alongside their corresponding components?

Answer 3

1. Which files are components and which files are spec is not always obvious.
2. Tests are always directly adjacent to the files they apply to.
3. Unrelated tests are less likely to share a folder.
4. Tests must have the correct naming patterns to be recognised, i.e., *.test.js
5. Possible to isolate tests based on name patterns, i.e., user-*

Question 4

What are the two most important Jest globals?

Answer 4

describe and it.

Question 5

What is the Jest global it?

Answer 5

it is a method that you pass a function as an argument, and whatever that function is, it will be run as though it’s a test by Jest.

Question 6

What is the Jest global it also known as?

Answer 6

The TEST global.

Question 7

What is the Jest global describe also known as?

Answer 7

The SUITE global.

Question 8

What is the Jest global describe?

Answer 8

An optional method for grouping any number of it or TEST statements.

Question 9

What happens in “watch mode”?

Answer 9

Tests are run automatically as files change.

Question 10

What tests are re-run in watch mode?

Answer 10

Only the ones pertaining to changed files.

Question 11

What does the BeforeEach global do?

Answer 11

Runs a block of code before each and every one of your tests.

Question 12

What is the BeforeEach global useful for?

Answer 12

Setting up databases, mock instances, etc.

Question 13

What does the BeforeAll global do?

Answer 13

Runs a block of code only once before the tests inside the suit are run.

Question 14

What does the AfterEach global do?

Answer 14

Runs a block of code after each test.

Question 15

What does the AfterAll global do?

Answer 15

Runs a block of code after the last test inside the suit.

Question 16

What are the AfterEach and AfterAll globals useful for?

Answer 16

Closing open connections, terminating sub-processes, etc.

Question 17

Which one runs first — beforeEach or beforeAll?

Answer 17

beforeAll.

Question 18

What does skipping a test result in?

Answer 18

That test not being run.

Question 19

What does isolating a test result in?

Answer 19

Only that and any other isolated tests running.

Question 20

How do you isolate a test?

Answer 20

By using it.only or fit to describe it rather than just it.

Question 21

How do you skip a test?

Answer 21

By using it.skip or xit to describe it rather than just it.

Question 22

What makes an asynchronous test different?

Answer 22

An asynchronous test doesn’t complete instantaneously and can take varying amounts of time. Jest must be notified when the test is complete.

Question 23

How are the three ways that we can notify Jest that an asynchronous test has reached a conclusion?

Answer 23

1. Invoke the done() callback that is passed to the test.
2. Return a promise from the test.
3. Pass an async function to describe.

Question 24

What’s the default timeout window for asynchronous Jest tests?

Answer 24

5 second or 5000 milliseconds.

Question 25

In what environment is running vs watching tests is useful?

Answer 25

Running tests is most useful for CI suites and watching tests is most useful for development.