Unit Testing

TL;DR

Unit Testing involves testing the smallest pieces of your code (usually individual functions or classes) in complete isolation from the rest of the application and external dependencies (like databases or networks).

Mental Model

How It Works

A good unit test follows the AAA pattern:

1. Arrange: Set up the inputs and initial state.
2. Act: Call the function being tested.
3. Assert: Verify that the output matches the expected result.

Frameworks like Jest, Mocha, or the native Node.js node:test module provide the test runner and assertion libraries.

Example

// math.js
function add(a, b) {
    if (typeof a !== 'number' || typeof b !== 'number') {
        throw new Error('Inputs must be numbers');
    }
    return a + b;
}
module.exports = { add };

// math.test.js (Using Jest)
const { add } = require('./math');

describe('add() function', () => {
    it('should correctly add two positive numbers', () => {
        // Arrange
        const a = 5, b = 10;
        // Act
        const result = add(a, b);
        // Assert
        expect(result).toBe(15);
    });

    it('should throw an error if input is a string', () => {
        // Asserting errors requires wrapping the act in a callback
        expect(() => add(5, "10")).toThrow('Inputs must be numbers');
    });
});

Common Interview Questions

Should a Unit Test connect to a database?

No. If a test connects to a database, reads a file, or makes an HTTP request, it is an Integration Test, not a Unit Test. Unit tests must be completely deterministic and run in milliseconds.

What is Code Coverage?

Code Coverage is a metric that shows what percentage of your source code lines were executed during the unit tests. While 100% coverage is nice, it does not guarantee your code is bug-free (it only means every line ran, not that every logical edge case was asserted).