Jest property testing demo

Run

# Install all dependencies
npm install # or:
yarn install

# Run tests and then exit immediately
npm test # or
yarn test

# Run tests in watch mode, with auto-re-run, and interactive prompt
npm test -- --watch # or
yarn test -- --watch

Let's inspect the snapshots to see what these are.

• Open __snapshots__/math.ppty.jest.js.snap and see what the failures are.
• Looks like there are some failures that occur when one of the parameters is null.
• Now, the snapshots capture the incorrect behaviour.

So let's fix the system under test!

• Edit math.js, and uncomment the FIXME 1 section.
• If Jest is running in watch mode, it should re-run the tests now, and there should be some snapshot failures.
• Hit the u key to update snapshots.
• Now, the snapshots capture the correct behaviour.

How it works

TestCheck brings QuickCheck style property based testing to Javascript. However, it does not give you the ability to save generated test cases that have previously failed as example based tests.

Jest is a testing framework that has the ability to save and update snapshots, which are programmatically serialised test expectations.

This demo uses snapshots in a creative manner to persist (the seeds required to reproduce) failed generated test cases, thereby generating example based test cases from them - achieving the original objective, of saving the failing test cases generated from properties.

Reading material

The idea for this came about from the following:

• QuickCheck
• What is property
• Producing example based tests from bulk tests

Author

Brendan Graetz

Licence

GPL-3.0