javascript-unit-testing

JavaScript Unit Testing

Expert guidance for writing maintainable, trustworthy unit tests in JavaScript and TypeScript using Jest. Based on "The Art of Unit Testing, Third Edition" by Roy Osherove with Vladimir Khorikov (Manning, 2024).

Instructions

When helping users with unit testing:

1. Understand the context: Identify if they're writing new tests, fixing existing ones, or learning concepts
2. Apply core principles: Focus on readability, maintainability, and trust in tests
3. Use appropriate patterns: Select the right testing pattern based on exit point type (return value, state change, or third-party call)
4. Provide examples: Show concrete code examples following best practices
5. Reference supporting documentation: Point to references/REFERENCE.md for detailed concepts and references/EXAMPLES.md for more code samples

Core Concepts

Unit of Work

A unit of work is all actions between an entry point (function/method we trigger) and one or more exit points (observable results).

Three types of exit points:

1. Return value - Function returns a useful value
2. State change - Observable change in system state
3. Third-party call - Calling external dependency (logger, database, API)

Good Unit Test Properties

Must have:

• Fast execution (milliseconds)
• Fully isolated from other tests
• Consistent results (no flakiness)
• Runs in memory (no filesystem, network, database)
• Clear intent and easy to read

Avoid:

• Logic in tests (if/else, loops, try/catch)
• Multiple concerns per test
• Shared state between tests
• Testing implementation details vs behavior

Quick Start: Writing Your First Test

Basic Test Structure (AAA Pattern)

test('sum with two numbers returns their sum', () => {
  // Arrange - set up test data
  const input = '1,2';

  // Act - call the unit of work
  const result = sum(input);

  // Assert - verify the outcome
  expect(result).toBe(3);
});