What and Why?

In 1969, we landed people on the moon with just 64KB of memory and a 43KHz CPU, yet in 2024, I can't run a simple Jest suite without maxing out my RAM and CPU, and waiting forever for it to finish.

This package helps you identify Jest's performance bottlenecks and, most importantly, boosts its performance right out of the box.

How to install

This package supports [email protected].

1. npm i jest-neat-runner
2. Make following changes in your jest.config.js file

module.exports = {
  globals: {
    NEAT_RUNTIME_CACHE: true,
    NEAT_TRANSFORM_CACHE: true,
  },
  runner: './node_modules/jest-neat-runner/build/runner',
  runtime: './node_modules/jest-neat-runner/build/runtime'
};

That's it!

The first run will be slightly faster, but the real power of this library kicks in during subsequent runs, as it utilizes the created runtime cache.

Features

Put the configuration below into the globals field in the Jest config.

Performance Boost

NEAT_RUNTIME_CACHE true/false
Caches runtime dependencies (tree-shaking). This resolves issues with barrel files and other unused modules that are unnecessarily loaded in your project.

NEAT_TRANSFORM_CACHE true/false
Improves caching optimization for already transformed modules. It transforms the file and keeps the cache in memory.

NEAT_MODULES_WITH_SIDE_EFFECTS ['module-name', 'lodash']
List of modules that contain side effects, such as modifying global objects or other actions that break runtime execution.

Debugging

NEAT_DEBUG true/false
Prevents cache invalidation on a failed test, allowing you to rerun the test, check the stack trace, and identify the root cause.

NEAT_REPORT_MODULE true/false
Prints the total module count and load times.

NEAT_REPORT_TRANSFORM true/false
Prints a table with information for each file suffix, showing the number of files with each extension and how long they took to transform (compile).

NEAT_REPORT_MODULE_LOAD_ABOVE_MS 200
Prints modules that took longer than the specified number of milliseconds to load.

NEAT_REPORT_ONLY_NODE_MODULES true/false
Works alongside NEAT_REPORT_MODULE_LOAD_ABOVE_MS to print only the modules from the node_modules folder.

Troubleshooting

Tests Are Still Slow

Add NEAT_DEBUG to the Jest config. This will prevent cache invalidation and it won't rerun the test in case of failure. Check the error message to identify the module causing the failure. Most of the time, it’s due to modules that contain side effects (e.g., mutating global scopes). When you find the culprit, add it to the NEAT_MODULES_WITH_SIDE_EFFECTS array in the Jest config's globals field.

"jest_neat_runner" Warning in the Console

Follow the steps mentioned above.

Production Readiness

Both the runner and runtime are not drop-in replacements for the original ones. Instead, they decorate the original jest-runner and jest-runtime. With a bit of luck, no breaking changes are expected.