The Art of Writing Better Tests: 9 Best Practices for JavaScript Developers

Test Anatomy and Descriptions: The Foundation of Good Testing

A well-structured test is easy to read and understand. To achieve this, you can use the AAA pattern, which stands for Arrange, Act, and Assert. This pattern helps break down the logic inside tests into three parts, making them more manageable and easier to comprehend.

describe('Example Test', () => {
  let result;

  // Arrange
  beforeEach(() => {
    result = [];
  });

  // Act
  it('adds items to the result array', () => {
    result.push('item1');
    result.push('item2');
  });

  // Assert
  it('contains two items', () => {
    expect(result.length).toBe(2);
  });
});

Another crucial aspect of test anatomy is writing detailed test descriptions. A simple yet effective approach is to use a three-layer system:

• Layer 1: Identify the unit or requirement being tested
• Layer 2: Describe the specific action or scenario being tested
• Layer 3: Outline the expected result

Unit Testing Anti-Patterns: What to Avoid

When it comes to unit testing, there are some common mistakes to avoid. One of the most significant errors is testing private methods. Instead, focus on testing public methods, which will cover the implementation details of private methods.

// Avoid testing private methods
it('calls privateMethod', () => {
  const privateMethodSpy = jest.spyOn(MyClass, 'privateMethod');
  MyClass.publicMethod();
  expect(privateMethodSpy).toHaveBeenCalledTimes(1);
});

// Instead, test public methods
it('calls publicMethod', () => {
  const result = MyClass.publicMethod();
  expect(result).toBe('expected result');
});

Another anti-pattern is catching errors in tests using try…catch statements. This approach can lead to false positives and make it difficult to identify real issues. Instead, use built-in functions like Jest’s toThrow function to expect errors.

// Avoid using try...catch statements
it('throws an error', () => {
  try {
    MyClass.publicMethod();
  } catch (error) {
    expect(error).toBeInstanceOf(Error);
  }
});

// Instead, use Jest's toThrow function
it('throws an error', () => {
  expect(() => MyClass.publicMethod()).toThrow(Error);
});

Test Preparation: Setting the Stage for Success

Test preparation is critical for creating effective tests. One common mistake is overusing helper libraries, which can lead to confusion and complexity. Remember, testing should be easy and fun! Aim to keep your testing setup simple and straightforward.

Another important consideration is the use of test preparation hooks like beforeAll and beforeEach. While these hooks can be useful, they can also create complexity and make it difficult to debug issues. Use them sparingly and only when necessary.

// Avoid using beforeAll and beforeEach unnecessarily
beforeAll(() => {
  // setup code
});

beforeEach(() => {
  // setup code
});

// Instead, use them sparingly and only when necessary
describe('Example Test', () => {
  let result;

  beforeEach(() => {
    result = [];
  });

  // Tests
});