Jest: Run Only Affected Tests

In modern software development, especially in large projects, running all tests on every change can become a significant bottleneck. Identifying and executing only the tests that are affected by a code modification dramatically speeds up the development feedback loop, allowing developers to iterate more quickly and confidently.

This challenge focuses on implementing a mechanism to identify and run Jest tests that are impacted by recent code changes.

Problem Description

You are tasked with creating a Jest test runner utility that, given a list of changed files, identifies and executes only those Jest tests that are potentially affected by these changes. This involves analyzing the dependencies between code files and their corresponding test files.

What needs to be achieved:

1. Dependency Mapping: Create a system that can map code files to the test files that cover them.
2. Affected Test Identification: Based on a list of changed files, determine which test files are potentially affected.
3. Selective Test Execution: Use Jest's programmatic API to run only the identified affected tests.

Key requirements:

• The solution should accept a list of file paths representing changed files.
• It should identify which Jest test files (.spec.ts, .test.ts, .spec.js, .test.js within a specified directory) cover the changed files.
• It should then execute these identified affected tests using Jest.
• The output should clearly indicate which tests were run.
• The solution should be implemented in TypeScript.

Expected behavior:

When provided with a list of changed files, the utility should:

1. Scan the project to build a dependency map (e.g., filePath -> [dependentTestFilePaths]).
2. For each changed file, find all its dependent test files using the map.
3. Compile a unique list of all affected test files.
4. Invoke Jest to run tests only within these affected files.
5. Report the outcome of the affected tests (e.g., success/failure counts).

Important edge cases to consider:

• No changes: What happens if the list of changed files is empty?
• Non-test files changed: What if a file that is not a source file (e.g., a config file, or a file not covered by any tests) is changed?
• Multiple changes: How does the system handle multiple files being changed simultaneously?
• Dynamic imports/require: While a full AST analysis might be complex for this challenge, consider how a simplified approach might handle common import/require patterns. For this challenge, we will focus on a basic import/require analysis.
• File naming conventions: Ensure the solution correctly identifies test files based on common conventions.

Examples

Example 1: Basic Dependency

Let's assume a project structure:

src/
  utils.ts
  math.ts
tests/
  utils.spec.ts
  math.spec.ts
  app.spec.ts

And the following import relationships (simplified):

• tests/utils.spec.ts imports src/utils.ts
• tests/math.spec.ts imports src/math.ts
• tests/app.spec.ts imports src/utils.ts and src/math.ts

Input:

Changed files: ['src/utils.ts']

Output (Conceptual):

Running affected tests for: ['src/utils.ts']
Affected test files: ['tests/utils.spec.ts', 'tests/app.spec.ts']
[Jest output for tests/utils.spec.ts and tests/app.spec.ts]

Explanation: Since src/utils.ts was changed, the tests that import it (tests/utils.spec.ts and tests/app.spec.ts) are considered affected and will be run. tests/math.spec.ts will not be run.

Example 2: Multiple Changes

Using the same project structure as Example 1.

Input:

Changed files: ['src/utils.ts', 'src/math.ts']

Output (Conceptual):

Running affected tests for: ['src/utils.ts', 'src/math.ts']
Affected test files: ['tests/utils.spec.ts', 'tests/math.spec.ts', 'tests/app.spec.ts']
[Jest output for tests/utils.spec.ts, tests/math.spec.ts, and tests/app.spec.ts]

Explanation: Both src/utils.ts and src/math.ts were changed. This affects tests/utils.spec.ts, tests/math.spec.ts, and tests/app.spec.ts. All unique affected test files will be run.

Example 3: Non-source file change (not covered by tests)

Input:

Changed files: ['README.md']

Output (Conceptual):

Running affected tests for: ['README.md']
No affected test files found.
[Jest output indicating no tests were run, or a success message]

Explanation: README.md is not a source file and not imported by any tests. Therefore, no tests are considered affected.

Constraints

• Project Scope: Assume the project has a standard directory structure where source code is in src/ and tests are in tests/.
• Test File Naming: Test files will follow the convention *.spec.ts, *.test.ts, *.spec.js, *.test.js.
• Dependency Analysis: For simplicity, you can assume a basic dependency analysis based on string matching of import and require statements within test files. A full Abstract Syntax Tree (AST) analysis is not required for this challenge, but a robust string-matching approach is encouraged.
• Jest Integration: You will need to use Jest's programmatic API (jest-cli or jest/core) to execute tests.
• TypeScript: The solution must be written in TypeScript.
• File System Access: The solution will require read access to the file system to scan directories and read file content.
• No External Testing Framework: Do not use other testing frameworks or libraries to manage test execution. Jest itself is the tool for running tests.

Notes

• Consider how you will build the dependency map. A simple approach might involve:
  1. Walking the src/ directory to identify source files.
  2. Walking the tests/ directory to identify test files.
  3. For each test file, read its content and parse import and require statements to find which source files it depends on.
  4. Invert this mapping to get sourceFile -> [testFiles].
• When identifying affected tests, remember to collect a unique set of test files. A test file might depend on multiple changed source files, but it should only be added to the execution list once.
• The programmatic API for Jest allows you to pass an array of file paths to runCLI or similar functions, enabling selective test execution.
• Think about how to handle relative vs. absolute import paths in your dependency analysis. For this challenge, assume standard relative paths within the project.
• The goal is to demonstrate the concept of identifying affected tests. A perfect, production-ready dependency resolver with AST analysis is beyond the scope, but a reasonably accurate string-based parser will be sufficient.