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();
}); Detection [x]Automatic You can detect this smell by identifying tests that rely on external files instead of generating or mocking the data. Look for file path references and check if they are necessary. Tags Testing Level [x]Intermediate AI Generation AI generators might create this smell if they aren't given clear instructions to generate or mock test data instead of using external files. AI Detection GPT tools detect this smell if you guide them to check the code for external file dependencies. Conclusion Never use existing files and keep your tests runnable to a known state. You need to generate your test data either by the test or mock it out completely so that tests are in full control. Relations Code Smell 30 - Mocking Business Code Smell 254 - Mystery Guest Code Smell 52 - Fragile Tests Code Smell 204 - Tests Depending on Dates More Info Coupling - The one and only software design problem Disclaimer Code Smells are my opinion. Credits Photo by William Warby on Unsplash Code without tests is broken by design. Jacob Kaplan-Moss Software Engineering Great Quotes This article is part of the CodeSmell Series. How to Find the Stinky Parts of your Code Don't rely on things that can change without you noticing them Don't rely on things that can change without you noticing them TL;DR: Tests must be in full control. TL;DR: Tests must be in full control. Problems Unreliable tests Difficult debugging Unexpected changes Coupling to External dependencies Mistery Guests Flaky tests Slowness Unreliable tests Difficult debugging Unexpected changes Coupling to External dependencies Coupling Mistery Guests Mistery Guests Flaky tests Flaky Slowness Solutions Generate the file in the test Mock it Use hardcoded streams instead Generate the file in the test Mock it Mock 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 . 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');
}); 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();
}); 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();
}); Detection [x]Automatic [x] Automatic [x] You can detect this smell by identifying tests that rely on external files instead of generating or mocking the data. Look for file path references and check if they are necessary. Tags Testing Testing Level [x]Intermediate [x] Intermediate [x] AI Generation AI generators might create this smell if they aren't given clear instructions to generate or mock test data instead of using external files. AI Detection GPT tools detect this smell if you guide them to check the code for external file dependencies. Conclusion Never use existing files and keep your tests runnable to a known state. You need to generate your test data either by the test or mock it out completely so that tests are in full control. Relations Code Smell 30 - Mocking Business Code Smell 30 - Mocking Business Code Smell 254 - Mystery Guest Code Smell 254 - Mystery Guest Code Smell 52 - Fragile Tests Code Smell 52 - Fragile Tests Code Smell 204 - Tests Depending on Dates Code Smell 204 - Tests Depending on Dates More Info Coupling - The one and only software design problem Coupling - The one and only software design problem Disclaimer Code Smells are my opinion . opinion Credits Photo by William Warby on Unsplash William Warby Unsplash Code without tests is broken by design. Code without tests is broken by design. Jacob Kaplan-Moss Jacob Kaplan-Moss Software Engineering Great Quotes Software Engineering Great Quotes This article is part of the CodeSmell Series. How to Find the Stinky Parts of your Code How to Find the Stinky Parts of your Code

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