Test lifecycle hooks (before, after) in Node.js

Introduction

Test lifecycle hooks help you run code before or after your tests. This keeps tests clean and avoids repeating setup or cleanup steps.

You want to prepare data or environment before running tests.

You need to reset or clean resources after tests finish.

You want to open a database connection before tests and close it after.

You want to initialize variables or mock functions once before tests.

You want to log or report results after all tests run.

Syntax

Node.js

before(() => {
  // code to run once before all tests
});
after(() => {
  // code to run once after all tests
});
beforeEach(() => {
  // code to run before each test
});
afterEach(() => {
  // code to run after each test
});

before runs once before all tests in a file or suite.

after runs once after all tests finish.

beforeEach and afterEach run before and after each individual test.

Examples

This runs once before any test starts.

Node.js

before(() => {
  console.log('Setup before all tests');
});

This runs once after all tests finish.

Node.js

after(() => {
  console.log('Cleanup after all tests');
});

These run before and after every single test.

Node.js

beforeEach(() => {
  console.log('Runs before each test');
});
afterEach(() => {
  console.log('Runs after each test');
});

Sample Program

This example shows how before sets the initial count, beforeEach increments it before each test, and afterEach logs the count after each test. after runs once after all tests.

Node.js

import { describe, it, before, after, beforeEach, afterEach } from 'mocha';
import assert from 'assert';

let count = 0;

describe('Counter tests', () => {
  before(() => {
    console.log('Starting tests');
    count = 10; // setup before all tests
  });

  after(() => {
    console.log('Tests finished');
  });

  beforeEach(() => {
    count += 1; // increment before each test
  });

  afterEach(() => {
    console.log(`Count after test: ${count}`);
  });

  it('should be 11 in first test', () => {
    assert.strictEqual(count, 11);
  });

  it('should be 12 in second test', () => {
    assert.strictEqual(count, 12);
  });
});

Important Notes

Use lifecycle hooks to avoid repeating setup or cleanup code in every test.

Hooks run in the order: before -> beforeEach -> test -> afterEach -> after.

If a hook fails, it can stop tests from running, so keep hook code simple and reliable.

Summary

Lifecycle hooks help prepare and clean up around tests.

before and after run once; beforeEach and afterEach run around every test.

They keep tests organized and avoid repeated code.

Practice

1. What is the main purpose of the before hook in Node.js testing frameworks like Mocha?

easy

A. To run setup code once before all tests in a suite

B. To run cleanup code after each test

C. To run code before each individual test

D. To skip a test conditionally

Solution

1. Step 1: Understand the role of before hook

   The before hook runs once before all tests in a test suite to prepare the environment.

2. Step 2: Differentiate from other hooks

   beforeEach runs before each test, and after runs after all tests, so they do not match the question.

3. Final Answer:

   To run setup code once before all tests in a suite -> Option A

4. Quick Check:

   before = setup once before all tests [OK]

Hint: Remember: before = once before all tests [OK]

Common Mistakes:

• Confusing before with beforeEach
• Thinking before runs after tests
• Mixing before with afterEach

2. Which of the following is the correct syntax to define a beforeEach hook in Mocha?

easy

A. before_each(() => { /* setup code */ });

B. beforeEach(() => { /* setup code */ });

C. beforeEach {}

D. beforeEach: function() { /* setup code */ }

Solution

1. Step 1: Recall Mocha hook syntax

   Mocha uses functions like beforeEach(() => { ... }) to define hooks.

2. Step 2: Check each option

   beforeEach(() => { /* setup code */ }); uses correct arrow function syntax. before_each(() => { /* setup code */ }); uses invalid function name. beforeEach {} is incomplete syntax. beforeEach: function() { /* setup code */ } uses incorrect object notation.

3. Final Answer:

   beforeEach(() => { /* setup code */ }); -> Option B

4. Quick Check:

   Correct hook syntax uses function call with callback [OK]

Hint: Use camelCase and parentheses for hooks [OK]

Common Mistakes:

• Using underscores instead of camelCase
• Omitting parentheses
• Using object notation instead of function call

3. Consider this Mocha test code snippet:

let count = 0;
before(() => { count += 1; });
after(() => { count += 10; });
it('test 1', () => { count += 100; });
it('test 2', () => { count += 1000; });

What is the final value of count after all tests run?

medium

A. 1110

B. 1112

C. 111

D. 1111

Solution

1. Step 1: Calculate increments in order

   before runs once: count = 0 + 1 = 1.
   Then test 1: count = 1 + 100 = 101.
   Then test 2: count = 101 + 1000 = 1101.
   Finally after: count = 1101 + 10 = 1111.

2. Step 2: Confirm no other increments

   No beforeEach or afterEach hooks affect count, so final is 1111.

3. Final Answer:

   1111 -> Option D

4. Quick Check:

   Sum increments: 1 + 100 + 1000 + 10 = 1111 [OK]

Hint: Add increments in hook and test order [OK]

Common Mistakes:

• Adding after hook before tests
• Forgetting before hook runs once
• Confusing beforeEach with before

4. Given this Mocha test code:

before(() => { console.log('Setup'); });
after(() => { console.log('Cleanup'); });

it('test 1', () => { console.log('Test 1'); });

it('test 2', () => { console.log('Test 2'); });

What is the correct order of console output?

medium

A. Test 1, Test 2, Setup, Cleanup

B. Test 1, Setup, Test 2, Cleanup

C. Setup, Test 1, Test 2, Cleanup

D. Setup, Test 1, Cleanup, Test 2

Solution

1. Step 1: Understand hook execution order

   before runs once before all tests, so 'Setup' prints first.

2. Step 2: Tests run in order, then after hook

   Tests print 'Test 1' then 'Test 2'. After all tests, after runs printing 'Cleanup'.

3. Final Answer:

   Setup, Test 1, Test 2, Cleanup -> Option C

4. Quick Check:

   before -> tests -> after = Setup, Test 1, Test 2, Cleanup [OK]

Hint: before runs once first; after runs once last [OK]

Common Mistakes:

• Assuming after runs after each test
• Mixing test order
• Thinking before runs before each test

5. You want to open a database connection once before all tests and close it once after all tests. Which hook pair should you use in Mocha?

hard

A. before to open and after to close

B. beforeEach to open and afterEach to close

C. after to open and before to close

D. beforeEach to open and after to close

Solution

1. Step 1: Identify when to open connection

   Opening once before all tests means using before hook.

2. Step 2: Identify when to close connection

   Closing once after all tests means using after hook.

3. Final Answer:

   before to open and after to close -> Option A

4. Quick Check:

   Open once before all, close once after all [OK]

Hint: Use before/after for once-only setup/cleanup [OK]

Common Mistakes:

• Using beforeEach/afterEach causing repeated open/close
• Reversing before and after roles
• Mixing hooks for different purposes