Unit Testing

Unit tests are fast and focused tests that verify the behavior of individual functions or components in isolation. Unlike End-to-End (E2E) tests, which validate complete user flows, unit tests ensure that the building blocks of your codebase work as expected.

Purpose of Unit Testing

• Validates small, isolated units of logic (e.g., functions like sort and filter).
• Catches bugs early in the development cycle, before integration or deployment.
• Increases code quality and makes refactoring safer.
• Provides fast feedback during development.

Example: utils.js

In this example, we are testing two utility functions: sort and filter. These are used to sort and filter arrays of objects. The tests are written using Jest.

import { sort, filter } from '../utils.js';

const arr = [
  { name: 'C' },
  { name: 'A' },
  { name: 'B' },
];

sort function

What’s being tested:

• Sorting an array in ascending and descending order based on a key (e.g., name)..
• Ensures the original array remains unchanged by working on a copy.

    describe('sort', () => {
        it('sorts array ascending by key', () => {
          const result = sort([...arr], 'name', 'asc');
          expect(result.map(x => x.name)).toEqual(['C', 'B', 'A']);
        });
  
        it('sorts array descending by key', () => {
          const result = sort([...arr], 'name', 'desc');
          expect(result.map(x => x.name)).toEqual(['A', 'B', 'C']);
        });
      });

  Note: The expected output in the test might seem counterintuitive — “ascending” returns ['C', 'B', 'A']. This could be a logic bug or a naming mismatch in the actual implementation.

filter function

What’s being tested:

• Case-insensitive filtering based on a string match within a specific key.
• Returns the original array if the search term is empty or null.
• Returns an empty array if there are no matches.

  describe('filter', () => {
    it('filters array by key and term (case-insensitive)', () => {
      expect(filter(arr, 'name', 'a')).toEqual([{ name: 'A' }]);
      expect(filter(arr, 'name', 'B')).toEqual([{ name: 'B' }]);
    });

    it('returns original array if term is empty', () => {
      expect(filter(arr, 'name', '')).toEqual(arr);
      expect(filter(arr, 'name', null)).toEqual(arr);
    });

    it('returns empty array if no match', () => {
      expect(filter(arr, 'name', 'm')).toEqual([]);
    });
  });

How to Run Unit Tests

You can run all unit tests using this command:

npm run test:unit

npm run test:unit -- utils.spec.js

npm run test:unit -- --watch

Summary

• Unit tests are great for fast and focused feedback.
• They work best for logic-heavy parts of your code (pure functions, helpers).
• Pair them with integration and E2E tests for full confidence.