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
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
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 { 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
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More Info
Coupling: The one and only software design problem
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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
Maxi Contieri ・ Dec 28 ’20
#programming
#quotes
#software
This article is part of the CodeSmell Series.
How to Find the Stinky parts of your Code
Maxi Contieri ・ May 21 ’21
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