Smart Test Suite Reduction for Jest

Your team has a large Jest test suite that is becoming increasingly slow to run. To address this, you need to implement a mechanism to intelligently reduce the test suite during development, allowing developers to focus on a subset of tests relevant to their current changes. This will significantly speed up feedback loops and improve developer productivity.

Problem Description

The goal is to create a system that can identify and run a reduced set of Jest tests based on changes made to the codebase. When a developer makes changes to a file, only the tests that are directly or indirectly dependent on that file should be executed. This will prevent running the entire suite for minor code modifications.

Key Requirements:

• Dependency Tracking: The system must be able to track dependencies between test files and the source code files they cover.
• Selective Execution: Based on modified files, the system should intelligently select and execute only the relevant subset of tests.
• Integration with Jest: The solution should integrate seamlessly with Jest's test runner.
• Configuration: Allow for some level of configuration, such as specifying which files are considered test files and which are source files.

Expected Behavior:

When a developer modifies src/components/Button.tsx, the system should:

1. Identify src/components/Button.tsx as a modified file.
2. Determine which test files cover src/components/Button.tsx.
3. Execute only those identified test files using Jest.

Edge Cases:

• No Modified Files: If no files are modified, the system should execute the full test suite.
• Modified Test Files: If a test file itself is modified, all tests within that file should be executed.
• Shared Dependencies: If multiple source files are modified, tests dependent on any of those modified files should be run.
• Configuration Errors: Handle cases where the test file patterns or source file patterns are invalid.

Examples

Example 1:

Assume the following file structure and dependencies:

• src/utils/format.ts is covered by tests/utils/format.test.ts
• src/components/Button.tsx is covered by tests/components/Button.test.ts and tests/ui/common.test.ts
• src/services/api.ts is covered by tests/services/api.test.ts

Input: Developer modifies src/components/Button.tsx.

Output: Jest is executed with the following tests: tests/components/Button.test.ts and tests/ui/common.test.ts.

Explanation: Only the tests that cover the modified Button.tsx file are executed.

Example 2:

Input: Developer modifies src/utils/format.ts and src/services/api.ts.

Output: Jest is executed with the following tests: tests/utils/format.test.ts, tests/services/api.test.ts.

Explanation: Tests covering both modified files are executed.

Example 3:

Input: Developer modifies tests/components/Button.test.ts.

Output: Jest is executed with tests/components/Button.test.ts.

Explanation: If a test file itself is modified, it is always included in the execution.

Constraints

• The solution should leverage existing Jest functionality and potentially its programmatic API.
• The dependency tracking mechanism should be efficient and not introduce significant overhead.
• The system should be configurable via a jest.config.js or a similar configuration file.
• The solution should be implemented in TypeScript.

Notes

Consider how you might establish the dependency mapping. This could involve:

• File Naming Conventions: A common convention is *.test.ts or *.spec.ts for test files covering files in a src directory.
• Import Statements: Analyzing import statements within test files can reveal direct dependencies on source files.
• Static Analysis Tools: More advanced solutions might involve using AST (Abstract Syntax Tree) parsers to understand code structure and dependencies.

Your task is to design and potentially implement a Jest runner or a script that orchestrates this intelligent test suite reduction. Think about how you would hook into Jest's execution flow or use its programmatic API to achieve this.