Fundamentals & Core Concepts Interview Questions

Jest is a JavaScript testing framework developed by Facebook that focuses on simplicity. Key features include: 1) Zero config setup for most JavaScript projects, 2) Snapshot testing, 3) Built-in code coverage reports, 4) Isolated test execution, 5) Interactive mode for test development, 6) Powerful mocking system, 7) Support for async testing.

Jest setup involves: 1) Installing Jest using npm/yarn: 'npm install --save-dev jest', 2) Adding test script to package.json: { 'scripts': { 'test': 'jest' } }, 3) Creating test files with .test.js or .spec.js extensions, 4) Configuring Jest through jest.config.js if needed, 5) Setting up any required environment configurations.

A Jest test consists of: 1) describe blocks for grouping related tests, 2) it or test blocks for individual test cases, 3) expect statements for assertions. Example: describe('MyFunction', () => { it('should return correct value', () => { expect(myFunction()).toBe(true); }); });

Matchers are methods that let you test values in different ways. Common matchers include: 1) toBe() for exact equality, 2) toEqual() for deep equality, 3) toContain() for arrays/iterables, 4) toBeTruthy()/toBeFalsy() for boolean checks, 5) toMatch() for regex. Used with expect(): expect(value).matcher()

Async testing can be handled through: 1) Returning promises: return promise.then(), 2) Async/await: async () => { await result }, 3) Done callback: (done) => { process.then(done) }, 4) Resolves/rejects matchers: expect(promise).resolves.toBe(). Always ensure async operations complete before test ends.

Jest provides lifecycle hooks: 1) beforeAll() - runs once before all tests, 2) beforeEach() - runs before each test, 3) afterEach() - runs after each test, 4) afterAll() - runs once after all tests. Used for setup/cleanup operations. Example: setting up database connections, cleaning test data.

Jest provides multiple mocking approaches: 1) jest.fn() for function mocking, 2) jest.mock() for module mocking, 3) jest.spyOn() for monitoring function calls, 4) Manual mocks using __mocks__ directory, 5) Automatic mocking of node_modules. Mocks help isolate code for testing.

Snapshot testing: 1) Captures component/data structure output, 2) Saves as reference file, 3) Compares against future changes, 4) Helps detect unintended changes. Example: expect(component).toMatchSnapshot(). Useful for UI components and serializable data.

Test isolation involves: 1) Using beforeEach/afterEach for setup/cleanup, 2) Avoiding shared state between tests, 3) Mocking external dependencies, 4) Using describe blocks for context separation, 5) Resetting mocks between tests using jest.clearAllMocks().

Code coverage measures how much code is tested. Jest configuration: 1) Add --coverage flag to test command, 2) Configure coverage settings in jest.config.js, 3) Set coverage thresholds, 4) Specify files to include/exclude, 5) Generate coverage reports in different formats.

Different configurations through: 1) Multiple jest.config.js files, 2) Using --config flag, 3) Environment-specific configs, 4) Test setup files, 5) Custom test environments. Example: separating unit and integration test configs.

Custom matchers: 1) Created using expect.extend(), 2) Define matcher function with pass/message, 3) Can access expect context, 4) Support async matchers, 5) Can be shared across tests. Example: expect.extend({ toBeWithinRange(received, floor, ceiling) { ... } }).

Environment variables handled through: 1) process.env in tests, 2) .env files with jest-environment, 3) setupFiles configuration, 4) CLI arguments, 5) Custom environment setup. Consider security and isolation of test environments.

Organization best practices: 1) Group related tests in describe blocks, 2) Use clear test descriptions, 3) Follow naming conventions (.test.js/.spec.js), 4) Organize test files alongside source code, 5) Separate unit and integration tests.

Timeout handling: 1) Global timeout in jest.config.js, 2) Individual test timeouts using jest.setTimeout(), 3) Timeout option in test blocks, 4) Async test timeout handling, 5) Custom timeout messages. Default timeout is 5 seconds.

Common CLI options: 1) --watch for watch mode, 2) --coverage for coverage reports, 3) --verbose for detailed output, 4) --runInBand for sequential execution, 5) --testNamePattern for filtering tests. Can be configured in package.json scripts.

Test fixtures implementation: 1) Using beforeEach for setup, 2) Shared test data files, 3) Factory functions for test data, 4) Fixture cleanup in afterEach, 5) Module-level fixture definitions. Helps maintain consistent test data.

Mock functions provide: 1) Call tracking, 2) Return value specification, 3) Implementation replacement, 4) Behavior verification, 5) Async mock support. Example: const mock = jest.fn().mockReturnValue(value).

Test retry handling: 1) Configure retry attempts, 2) Implement retry logic, 3) Handle async retry scenarios, 4) Log retry attempts, 5) Analyze flaky test patterns. Use jest-retry for automated retries.

Test contexts provide: 1) Shared state within describe blocks, 2) Setup/teardown management, 3) Data sharing between tests, 4) Context-specific configurations, 5) Isolated test environments. Used through beforeAll/beforeEach hooks.

Advanced mocking includes: 1) Partial module mocking, 2) Mock implementation factories, 3) Complex return value sequences, 4) Mock instance methods, 5) Mock timers and events. Example: jest.spyOn() with complex implementations.

Complex async testing: 1) Multiple promise chains, 2) Parallel async operations, 3) Error handling scenarios, 4) Timeout management, 5) Race condition testing. Use async/await with proper error handling.

Custom environments: 1) Extending Jest's environment, 2) Implementing global setup/teardown, 3) Custom DOM implementation, 4) Environment-specific mocks, 5) Shared environment state. Used for specialized testing needs.

Advanced snapshots: 1) Custom serializers, 2) Dynamic snapshot matching, 3) Inline snapshots, 4) Snapshot migrations, 5) Conditional snapshot updates. Used for complex component testing.

Performance testing: 1) Execution time measurements, 2) Memory usage tracking, 3) Performance benchmarks, 4) Resource utilization tests, 5) Performance regression detection. Use custom metrics and assertions.