Jest Incremental Testing: Efficiently Running Affected Tests

Modern software development often involves frequent code changes. Running the entire test suite after every small modification can be time-consuming and inefficient. Incremental testing, also known as affected testing, aims to address this by only running tests that are potentially impacted by the recent code changes. This challenge will guide you in setting up and utilizing a basic form of incremental testing with Jest.

Problem Description

Your task is to implement a system where Jest runs only a subset of tests based on a predefined notion of "changed" files. For this challenge, we will simulate changes by identifying specific files that have been modified. You will then configure Jest to run tests that depend on these modified files.

What needs to be achieved:

1. Define "changed" files: You'll need a way to specify which files are considered modified. For this exercise, this will be a simple list of file paths.
2. Map dependencies: You'll need a mechanism to determine which test files are dependent on which source files.
3. Configure Jest to run affected tests: You'll need to run Jest in a way that filters tests based on the identified dependencies and changed files.

Key requirements:

• The solution should leverage Jest's capabilities.
• You should be able to provide a list of "changed" file paths.
• The output should clearly indicate which tests were run.

Expected behavior:

When provided with a list of changed files, Jest should execute only those test files that directly or indirectly import or depend on the changed source files.

Important edge cases to consider:

• What happens when no files are changed? (All tests should ideally run, or a specific message should indicate no changes).
• What happens when a changed file is not imported by any test? (No tests should run for that specific file).
• Handling of directory changes (though for this specific challenge, we'll focus on individual file changes).

Examples

Example 1:

• Project Structure:

  src/
    math.ts
    utils.ts
  tests/
    math.test.ts
    utils.test.ts
    general.test.ts
  

• src/math.ts: Contains add and subtract functions.

• src/utils.ts: Contains a formatString function.

• tests/math.test.ts: Imports and tests add and subtract.

• tests/utils.test.ts: Imports and tests formatString.

• tests/general.test.ts: Does not import from src/math.ts or src/utils.ts.

• Input: changedFiles = ['src/math.ts']

• Expected Output (Conceptual): Jest will run tests/math.test.ts. tests/utils.test.ts and tests/general.test.ts will not be run.

Example 2:

• Project Structure: Same as Example 1.
• Input: changedFiles = ['src/utils.ts']
• Expected Output (Conceptual): Jest will run tests/utils.test.ts. tests/math.test.ts and tests/general.test.ts will not be run.

Example 3:

• Project Structure: Same as Example 1.
• Input: changedFiles = ['src/math.ts', 'src/utils.ts']
• Expected Output (Conceptual): Jest will run tests/math.test.ts and tests/utils.test.ts. tests/general.test.ts will not be run.

Constraints

• The project will be a standard TypeScript project set up with Jest.
• You will be provided with a list of strings, where each string is a relative path to a "changed" file within the src directory.
• The dependency mapping will be based on import statements in test files referencing source files. For simplicity, assume direct imports.
• Performance is a consideration; the goal is to significantly reduce the number of tests run compared to a full suite execution.

Notes

• This challenge aims to simulate a core concept of incremental testing. In a real-world scenario, determining file dependencies is often automated by build tools or specialized libraries (e.g., using AST parsing to track imports). For this challenge, you can simplify this by making reasonable assumptions or using a very basic mapping.
• Consider how you might pass the list of changed files to Jest. Jest has command-line arguments and configuration options that might be useful.
• Think about how to achieve the dependency mapping. A simple approach might involve manually defining which tests depend on which source files, or you could explore simple string matching on import paths within test files.
• The jest-runner or similar libraries might offer advanced features for this, but try to solve it using core Jest features or a straightforward configuration first.