Fundamentals & Core Concepts Interview Questions

Mocha is a feature-rich JavaScript test framework running on Node.js and browser. Key features include: 1) Flexible test structure with describe/it blocks, 2) Support for asynchronous testing, 3) Multiple assertion library support, 4) Test hooks (before, after, etc.), 5) Rich reporting options, 6) Browser support, 7) Plugin architecture.

Setup involves: 1) Installing Mocha: npm install --save-dev mocha, 2) Adding test script to package.json: { 'scripts': { 'test': 'mocha' } }, 3) Creating test directory, 4) Choosing assertion library (e.g., Chai), 5) Creating test files with .test.js or .spec.js extension.

describe() is used to group related tests (test suite), while it() defines individual test cases. Example: describe('Calculator', () => { it('should add numbers correctly', () => { /* test */ }); }). They help organize tests hierarchically and provide clear test structure.

Mocha handles async testing through: 1) done callback parameter, 2) Returning promises, 3) async/await syntax. Example: it('async test', async () => { const result = await asyncOperation(); assert(result); }). Tests wait for async operations to complete.

Mocha provides hooks: 1) before() - runs once before all tests, 2) beforeEach() - runs before each test, 3) after() - runs once after all tests, 4) afterEach() - runs after each test. Used for setup and cleanup operations. Example: beforeEach(() => { /* setup */ });

Mocha works with various assertion libraries: 1) Node's assert module, 2) Chai for BDD/TDD assertions, 3) Should.js for BDD style, 4) Expect.js for expect() style. Example with Chai: const { expect } = require('chai'); expect(value).to.equal(expected);

Mocha offers various reporters: 1) spec - hierarchical test results, 2) dot - minimal dots output, 3) nyan - fun nyan cat reporter, 4) json - JSON test results, 5) html - HTML test report. Select using --reporter option or configure in mocha.opts.

Tests can be skipped/pending using: 1) it.skip() - skip test, 2) describe.skip() - skip suite, 3) it() without callback - mark pending, 4) .only() - run only specific tests. Example: it.skip('test to skip', () => { /* test */ });

Exclusive tests using .only(): 1) it.only() runs only that test, 2) describe.only() runs only that suite, 3) Multiple .only() creates subset of tests to run, 4) Useful for debugging specific tests. Example: it.only('exclusive test', () => { /* test */ });

Timeout handling: 1) Set suite timeout: this.timeout(ms), 2) Set test timeout: it('test', function(done) { this.timeout(ms); }), 3) Default is 2000ms, 4) Set to 0 to disable timeout, 5) Can be set globally or per test.

Test retries configured through: 1) this.retries(n) in test/suite, 2) --retries option in CLI, 3) Retries count for failed tests, 4) Useful for flaky tests, 5) Can be set globally or per test. Example: this.retries(3);

Common CLI options: 1) --watch for watch mode, 2) --reporter for output format, 3) --timeout for test timeout, 4) --grep for filtering tests, 5) --bail to stop on first failure, 6) --require for requiring modules. Example: mocha --watch --reporter spec

Dynamic tests created by: 1) Generating it() calls in loops, 2) Using test data arrays, 3) Programmatically creating describe blocks, 4) Using forEach for test cases, 5) Generating tests from data sources.

Root hook plugin: 1) Runs hooks for all test files, 2) Configured in mocha.opts or CLI, 3) Used for global setup/teardown, 4) Affects all suites, 5) Useful for shared resources. Example: --require ./root-hooks.js

File setup options: 1) Use before/after hooks, 2) Require helper files, 3) Use mocha.opts for configuration, 4) Implement setup modules, 5) Use root hooks plugin. Ensures proper test environment setup.

Config options include: 1) .mocharc.js/.json file, 2) package.json mocha field, 3) CLI arguments, 4) Environment variables, 5) Programmatic options. Control test execution, reporting, and behavior.

Custom reporters: 1) Extend Mocha's Base reporter, 2) Implement required methods, 3) Handle test events, 4) Format output as needed, 5) Register reporter with Mocha. Allows customized test reporting.

Organization practices: 1) Group related tests in describes, 2) Use clear test descriptions, 3) Maintain test independence, 4) Follow consistent naming, 5) Structure tests hierarchically. Improves maintainability.

Data management: 1) Use fixtures, 2) Implement data factories, 3) Clean up test data, 4) Isolate test data, 5) Manage data dependencies. Ensures reliable test execution.

Parallel execution: 1) Use --parallel flag, 2) Configure worker count, 3) Handle shared resources, 4) Manage test isolation, 5) Consider file-level parallelization. Improves test execution speed.

Advanced filtering: 1) Use regex patterns, 2) Filter by suite/test name, 3) Implement custom grep, 4) Use test metadata, 5) Filter by file patterns. Helps focus test execution.

Custom interfaces: 1) Define interface methods, 2) Register with Mocha, 3) Handle test definition, 4) Manage context, 5) Support hooks and suites. Allows custom test syntax.

Complex async handling: 1) Chain promises properly, 2) Manage async timeouts, 3) Handle parallel operations, 4) Control execution flow, 5) Implement proper error handling. Important for reliable async tests.

Suite composition: 1) Share common tests, 2) Extend test suites, 3) Compose test behaviors, 4) Manage suite hierarchy, 5) Handle shared context. Enables test reuse.

Event testing patterns: 1) Listen for events, 2) Verify event data, 3) Test event ordering, 4) Handle event timing, 5) Test error events. Important for event-driven code.