property-based-testing

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Property-based testing with fast-check (TypeScript/JavaScript) and Hypothesis (Python). Generate test cases automatically, find edge cases, and test mathematical properties. Use when user mentions property-based testing, fast-check, Hypothesis, generating test data, QuickCheck-style testing, or finding edge cases automatically.

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Sourcelaurigates/claude-plugins

Added on2026-02-17

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SKILL.md Content

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Property-Based Testing

Expert knowledge for property-based testing - automatically generating test cases to verify code properties rather than testing specific examples.

When to Use This Skill

Use this skill when...	Use another skill instead when...
Testing mathematical properties (commutative, associative)	Writing specific example-based unit tests
Testing encode/decode roundtrips	Setting up test runner configuration
Finding edge cases automatically	Doing E2E browser testing
Validating data transformations and invariants	Analyzing test quality or smells
Testing API contracts with generated data	Running mutation testing

Core Expertise

Property-Based Testing Concept

Traditional testing: Test specific examples
Property-based testing: Test properties that should hold for all inputs
Generators: Automatically create diverse test inputs
Shrinking: Minimize failing cases to simplest example
Coverage: Explore edge cases humans might miss

When to Use Property-Based Testing

Mathematical operations (commutative, associative properties)
Encoders/decoders (roundtrip properties)
Parsers and serializers
Data transformations
API contracts
Invariants and constraints

TypeScript/JavaScript (fast-check)

Installation

bash

# Using Bun
bun add -d fast-check

# Using npm
npm install -D fast-check

Basic Example

typescript

import { test } from 'vitest'
import * as fc from 'fast-check'

// Property-based test
test('reverse twice returns original - property based', () => {
  fc.assert(
    fc.property(
      fc.array(fc.integer()), // Generate random arrays of integers
      (arr) => {
        expect(reverse(reverse(arr))).toEqual(arr)
      }
    )
  )
})
// fast-check automatically generates 100s of test cases!

Key Generators (Quick Reference)

Generator	Description
`fc.integer()`	Any integer (with optional min/max)
`fc.nat()`	Natural numbers (>= 0)
`fc.float()` / `fc.double()`	Floating-point numbers
`fc.string()`	Any string (with optional length)
`fc.emailAddress()`	Email format strings
`fc.array(arb)`	Arrays of arbitrary type
`fc.record({...})`	Object with typed fields
`fc.boolean()`	Boolean values
`fc.constantFrom(...)`	Pick from options
`fc.tuple(...)`	Fixed-size tuples
`fc.oneof(...)`	Union types
`fc.option(arb)`	Value or null
`fc.date()`	Date objects

Common Properties to Test

Property	Pattern	Example
Roundtrip	`f(g(x)) = x`	encode/decode, serialize/parse
Idempotence	`f(f(x)) = f(x)`	sort, normalize, format
Commutativity	`f(a,b) = f(b,a)`	add, merge, union
Associativity	`f(f(a,b),c) = f(a,f(b,c))`	add, concat
Identity	`f(x, id) = x`	multiply by 1, add 0
Inverse	`f(g(x)) = x`	encrypt/decrypt

Configuration

typescript

fc.assert(property, {
  numRuns: 1000,      // Run 1000 tests (default: 100)
  seed: 42,           // Reproducible tests
  endOnFailure: true, // Stop after first failure
})

Preconditions

typescript

fc.pre(b !== 0) // Skip cases where b is 0

Python (Hypothesis)

Installation

bash

# Using uv
uv add --dev hypothesis

# Using pip
pip install hypothesis

Basic Example

python

from hypothesis import given, strategies as st

@given(st.lists(st.integers()))
def test_reverse_twice_property(arr):
    assert reverse(reverse(arr)) == arr
    # Hypothesis automatically generates 100s of test cases!

Key Strategies (Quick Reference)

Strategy	Description
`st.integers()`	Any integer (with optional bounds)
`st.floats()`	Floating-point numbers
`st.text()`	Any string (with optional size)
`st.binary()`	Byte strings
`st.lists(strat)`	Lists of given strategy
`st.sets(strat)`	Unique value sets
`st.dictionaries(k, v)`	Dictionaries
`st.booleans()`	Boolean values
`st.sampled_from(...)`	Pick from options
`st.tuples(...)`	Fixed-size tuples
`st.one_of(...)`	Union types
`st.dates()` / `st.datetimes()`	Date/time values
`st.builds(Class, ...)`	Build objects from strategies

Configuration

python

from hypothesis import given, settings, strategies as st

@settings(max_examples=1000, deadline=None)
@given(st.lists(st.integers()))
def test_with_custom_settings(arr):
    assert sort(arr) == sorted(arr)

Assumptions

python

from hypothesis import assume
assume(b != 0)  # Skip cases where b is 0

Stateful Testing

Hypothesis supports stateful testing via

RuleBasedStateMachine

for testing sequences of operations against invariants.

Agentic Optimizations

Context	Command
Quick TS test	`bunx vitest --dots --bail=1 --grep 'property'`
Quick Python test	`uv run pytest -x -q --tb=short -k 'property'`
CI TS test	`bunx vitest run --reporter=junit --grep 'property'`
CI Python test	`uv run pytest --hypothesis-show-statistics -q`
Reproducible	`fc.assert(prop, { seed: 42 })` or `@settings(derandomize=True)`
Fast iteration	`fc.assert(prop, { numRuns: 50 })` or `@settings(max_examples=50)`

For detailed examples, advanced patterns, and best practices, see REFERENCE.md.

References

fast-check: https://fast-check.dev/
Hypothesis: https://hypothesis.readthedocs.io/
Property-Based Testing: https://fsharpforfunandprofit.com/posts/property-based-testing/

property-based-testing

NPX Install

Tags

SKILL.md Content

Property-Based Testing

When to Use This Skill

Core Expertise

TypeScript/JavaScript (fast-check)

Installation

Basic Example

Key Generators (Quick Reference)

Common Properties to Test

Configuration

Preconditions

Python (Hypothesis)

Installation

Basic Example

Key Strategies (Quick Reference)

Configuration

Assumptions

Stateful Testing

Agentic Optimizations

See Also

References