Code Smell 259 - Control Your Environment to Avoid Test Failures

18 Jul 2024

Don't rely on things that can change without you noticing them

TL;DR: Tests must be in full control.

Problems

Unreliable tests
Difficult debugging
Unexpected changes
Coupling to External dependencies
Mistery Guests
Flaky tests
Slowness

Solutions

Generate the file in the test
Mock it
Use hardcoded streams instead

Context

Using an existing file for testing breaks the golden rule: tests must fully control their environment.

When someone changes the file, the test becomes erratic and unreliable.

That can result in an annoying debugging experience and non-deterministic test runs.

Sample Code

Wrong

const fs = require('fs');

function trimFile(data) {
    return data.trim();
}

// existing_file.txt holds the sample information
// "     John Wick    "

test('test process file', () => {
    const data = fs.readFileSync('existing_file.txt', 'utf8');
    expect(trimFile(data)).toBe('John Wick');
});

Right

const fs = require('fs');
const { jest } = require('@jest/globals');

function trimFile(data) {
    return data.trim();
}

function generateTestData() {
    return ' John Wick ';
}

test('test process file generated', () => {
    const data = generateTestData();
    expect(trimFile(data)).toBe('John Wick');
});

test('test process file mocked', () => {
    jest.spyOn(fs, 'readFileSync').mockReturnValue(' mocked data ');
    const data = fs.readFileSync('file.txt', 'utf8');
    expect(trimFile(data)).toBe('John Wick');
    fs.readFileSync.mockRestore();
});