fast-check-workshop

The purpose of this workshop is to introduce Property Based Testing with Fast-Check. The workshop is based off of the Kata on Chapter 6 of Property-Based Testing with PropEr, Erlang, and Elixir by Fred Hebert. I highly recommend this book.

The Kata

Given the following table:

Item	Unit Price	Special Price
A	50	3 for 130
B	30	2 for 45
C	20
D	15

Given a item price catalog, when given a list of items to purchase, then calculate the total. This project provides a sample skeleton, but feel free to adjust the code as needed.

Vague Requirements:

• Item names are unique
• List of items have no set length
• List of items are unordered
• Unit prices are whole numbers
• All items have a unit price
• Special prices can be applied multiple times
• Special prices are optional

Step 1: Identify the properties (5 minutes, +10 minute discussion)

Property based testing frameworks check the truthfulness of properties. A property is a statement like: for all (x, y, ...) such as precondition(x, y, ...) holds property(x, y, ...) is true. - fast-check docs

As a group, let's identify the properties of this function:

Things to look for

Here are some patterns from the Quick Theories documentation.

• Invariant Pattern "Some things never change"
• The inverse function pattern "There and back again"
• Idempotence "The more things change, the more they stay the same"
• Analogous function pattern "Different paths same destination"

Step 2: Creating generators (15 minutes, +10 minute discussion)

Working with the fast-check basic arbitraries, create a generator to generate a random item.

Step 3: Test the property & implement (15 minutes, +10 minute discussion)

Using the total.spec.ts, test a property that you defined in Step 1. If you finish early, test more properties.

• Try configuring fast-check to run through more or fewer examples.

Step 4: Negative Testing (15 minutes, +10 minute discussion)

• Identify very, very broad properties
• Apply very, very broad inputs to verify that the system doesn't crash