Jest Lifecycle

Setup and Teardown

You may have some setup work that need run before tests (eg seed a test database), or you may have some work that needs to happen after tests run (eg clear the test database, clean jsdom).

Jest providers helper functions to handle theses cases.

for every test

we can use beforeEach to runs some code before each test

let users

function setup() {
  console.log('setup')
  users = ['jane', 'bob']
}

beforeEach(() => {
  setup()
})

test('removing user', () => {
  expect(users).toHaveLength(2)
  users.pop()
  expect(users).toHaveLength(1)
})
test('adding user', () => {
  expect(users).toHaveLength(2)
  users.push('mark')
  expect(users).toHaveLength(3)
})

 PASS  ./test.js
  ✓ removing user (3ms)
  ✓ adding user (1ms)

  console.log test.js:5
    setup

  console.log test.js:5
    setup

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total

afterEach can be used to run after each test

one-time setup

beforeAll and afterAll run only once before any test in the file runs and after all tests have been run

scope

top levelbefore* and after* blocks are run for all the tests in the file they are declared

if you want to scope before* or after* blocks to specific test, declared them inside a describe

describe('group', () => {
  beforeEach(() => {
    console.log('called')
  })

  test('test', () => {})
})

test('another test', () => {})

 PASS  ./test.js
  ✓ another test
  group
    ✓ test (7ms)

  console.log test.js:3
    called

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        2.887s
Ran all test suites.

beforeEach has been called only once

multiple setup/teardown

beforeAll(() => {
  console.log('called')
})

beforeAll(() => {
  console.log('called too')
})

test('test', () => {})
test('another test', () => {})

 PASS  ./test.js
  ✓ test
  ✓ another test

  console.log test.js:2
    called

  console.log test.js:6
    called too

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total

all before* and after* functions can be called multiple times and are executed in the order they are declared (useful for libraries to register their functions without overriding user declared)

order of execution

all describe sequentially
- nested describe takes precedence over sibling
beforeAll
every test in order of declaration (despite the depth)
- beforeEach
- test
- afterEach
afterAll

beforeAll(() => {
  console.log('beforeAll')
})

beforeEach(() => {
  console.log('beforeEach')
})

afterAll(() => {
  console.log('afterAll')
})

afterEach(() => {
  console.log('afterEach')
})

describe('outer', () => {
  console.log('descibe outer')
  describe('inner', () => {
    console.log('descibe inner')
    test('inner', () => {
      console.log('outer.inner test')
    })
  })

  test('outer test', () => {
    console.log('outer test')
  })

  describe('inner 2', () => {
    console.log('descibe inner 2')
    test('inner 2', () => {
      console.log('outer.inner 2 test')
    })
  })
})

// descibe outer
// descibe inner
// descibe inner 2
// beforeAll
// beforeEach
// outer.inner test
// afterEach
// beforeEach
// outer test
// afterEach
// beforeEach
// outer.inner 2 test
// afterEach
// afterAll

skip and only

describe and test can make use of .skip to prevent the block from running or .only to skip all but this one

test('I will not run', () => {
  // ...
})
test.only('only me', () => {
  // ...
})

test.skip('I will not run', () => {
  // ...
})
test('I will run', () => {
  // ...
})
test('I will run too', () => {
  // ...
})

Author: Jaga Santagostino

Jest lifecycle