Jest Code Coverage with Cobertura Output

This challenge focuses on leveraging Jest to generate code coverage reports in the Cobertura XML format. Cobertura reports are a widely adopted standard for visualizing code coverage, making it easier to integrate with CI/CD pipelines and code quality tools. You will configure Jest to produce this specific output format and understand how to interpret the generated report.

Problem Description

Your task is to configure a Jest testing environment in a TypeScript project to generate code coverage reports in the Cobertura XML format. This involves setting up Jest's configuration to specify the desired output format and potentially writing a simple TypeScript function and a corresponding Jest test to demonstrate the coverage report generation.

Key Requirements:

1. Jest Configuration: Modify or create a jest.config.js (or jest.config.ts) file to instruct Jest to produce Cobertura reports.
2. Coverage Report Generation: Execute Jest in a way that generates the Cobertura XML file.
3. TypeScript Project: The solution should be for a TypeScript project.
4. Basic Functionality: Include a simple TypeScript function and a Jest test for it to ensure there's code to measure coverage against.

Expected Behavior:

• Running Jest should produce a cobertura-coverage.xml file (or a similarly named file, depending on Jest configuration) in the project's root or specified coverage directory.
• This XML file should adhere to the Cobertura report schema.
• The report should accurately reflect the code coverage of the provided TypeScript function by its Jest test.

Edge Cases to Consider:

• No Tests Executed: What happens if no tests are run, or if the tests don't cover any of the code?
• Multiple Files: How does the report handle coverage across multiple TypeScript files? (For this specific challenge, one file is sufficient to demonstrate the concept).

Examples

Example 1: Basic Coverage

Project Structure:

/src
  - calculator.ts
/tests
  - calculator.test.ts
jest.config.js

src/calculator.ts:

export function add(a: number, b: number): number {
  return a + b;
}

export function subtract(a: number, b: number): number {
  return a - b;
}

tests/calculator.test.ts:

import { add } from '../src/calculator';

describe('Calculator', () => {
  test('adds two numbers', () => {
    expect(add(1, 2)).toBe(3);
  });
});

jest.config.js:

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  collectCoverage: true,
  coverageReporters: ['cobertura'],
  coverageDirectory: './coverage',
};

Execution:

Run npx jest --coverage

Expected Output (Simplified coverage/cobertura-coverage.xml snippet):

<?xml version="1.0" ?>
<coverage branch-rate="0" branches-covered="0" branches-valid="0" complexity="0" line-rate="0.5" lines-covered="1" lines-valid="2" timestamp="1678886400" version="6.0.0">
	<sources>
		<source>/path/to/your/project</source>
	</sources>
	<packages>
		<package branch-rate="0" complexity="0" line-rate="0.5" name="src">
			<classes>
				<class branch-rate="0" complexity="0" filename="src/calculator.ts" line-rate="0.5" name="calculator">
					<methods/>
					<lines>
						<line hits="1" number="2"/>
						<line hits="0" number="6"/>
					</lines>
				</class>
			</classes>
		</package>
	</packages>
</coverage>

Explanation:

The jest.config.js is configured to collect coverage and use the cobertura reporter. When npx jest --coverage is run, Jest analyzes the calculator.ts file and its tests. The add function is covered (hits="1"), while subtract is not (hits="0"), resulting in a line-rate of 0.5 for the calculator.ts file.

Constraints

• The project must be set up for TypeScript development (e.g., using ts-jest).
• Jest must be installed as a development dependency.
• The generated Cobertura report should be at least parsable by standard Cobertura viewers/tools.

Notes

• You will need to install jest, ts-jest, and @types/jest as development dependencies.
• The coverageReporters option in jest.config.js is crucial for specifying the output format.
• You can use npx jest --coverage to run Jest and generate coverage reports. The --coverage flag is shorthand for enabling coverage collection.
• Explore the generated cobertura-coverage.xml file to understand how lines of code are tracked and whether they were executed by tests.
• Consider how you might integrate this report into a CI/CD pipeline.