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

1. Generate the file in the test

2. Mock it

3. 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

Disclaimer

Code Smells are my opinion.

Credits

Photo by William Warby on Unsplash

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Code without tests is broken by design.

Jacob Kaplan-Moss

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This article is part of the CodeSmell Series.