global-leaks-finder

Global Leaks finder

A mocha-based testing tool to help find tests which are leaking globals.

The aim is to help catch and avoid tests with unexpected side-effects.

It does add significant time to the job so best run only occasionally. See below for explanation how it works.

Why?

Take this example. Before each test the browser is set and after it is reset. However we do not know what the value was before this unit test ran or if false is a reliable value for later tests. This is an example of a global leakage and is easily missed.

beforeEach(() => {
  process.browser = true;
});
afterEach(() => {
  process.browser = false; // BAD
});

Running the tool on this test would throw an error to the console Error: Global has changed on the test spec which has leaked, allowing you to catch and manage the problem. The test has now been improved.

let cached;
beforeEach(() => {
  cached = process.browser;
  process.browser = true;
});
afterEach(() => {
  process.browser = cached; // BETTER
});

Ideally its best to use something like Sinon.js Sandboxing, but it is not always possible.

Install

Does not require any additional dependencies, just itself.

npm install global-leaks-finder

Usage

mocha --check-leaks node_modules/global-leaks-finder/index.js <your test files here>

Mocha's check-leaks options

I recommend running in conjunction with Mocha's check-leaks flag. It compares against a small list of non-enumerable globals (i.e. global.newVariable = 'some-value'; would be caught) but would not catch something like process.execPath = 'GLOBAL LEAK'; which this tool would catch.

How does it work

It runs a global beforeEach and afterEach. For each test it hashes the globals at the start, hashes the globals at the end, and compares the 2 failing if the hash has changed. Hence why it has a hefty performance hit, but hopefully does its job.

Issues

Please feel free to contact or email me if there are any issues.