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Agent skill
Property Testing
Property-based testing with fast-check for business logic validation
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163
Forks
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Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/property-testing
SKILL.md
Property Testing
LivestockAI uses property-based testing (PBT) with fast-check to validate business logic invariants.
What is Property Testing?
Instead of testing specific examples, property tests verify that properties hold for ALL possible inputs:
typescript
// Example-based test
it('calculates FCR correctly', () => {
expect(calculateFCR(150, 100)).toBe(1.5)
})
// Property-based test
it('FCR is always positive when inputs are positive', () => {
fc.assert(
fc.property(
fc.float({ min: 0.1, max: 10000 }),
fc.float({ min: 0.1, max: 10000 }),
(feed, weight) => {
const fcr = calculateFCR(feed, weight)
return fcr === null || fcr > 0
},
),
)
})
fast-check Basics
typescript
import { describe, it, expect } from 'vitest'
import * as fc from 'fast-check'
describe('Property Tests', () => {
it('property holds for all inputs', () => {
fc.assert(
fc.property(fc.integer({ min: 1, max: 100000 }), (quantity) => {
// Property must return true or throw
return quantity > 0
}),
{ numRuns: 100 },
)
})
})
Common Arbitraries
typescript
// Integers
fc.integer({ min: 1, max: 100000 })
fc.nat() // Non-negative integer
// Floats
fc.float({ min: 0, max: 10000 })
// Strings
fc.string()
fc.uuid()
// Arrays
fc.array(fc.integer(), { minLength: 0, maxLength: 20 })
// Objects
fc.record({
quantity: fc.integer({ min: 1, max: 1000 }),
price: fc.float({ min: 0, max: 10000 }),
})
Inventory Invariant Example
From tests/features/batches/batches.property.test.ts:
typescript
/**
* Property 4: Inventory Invariant
* For any batch, current_quantity SHALL always equal:
* initial_quantity - sum(mortality) - sum(sales)
*/
describe('Property 4: Inventory Invariant', () => {
it('current_quantity equals initial - mortalities - sales', () => {
fc.assert(
fc.property(
fc.integer({ min: 1, max: 100000 }),
fc.array(fc.integer({ min: 1, max: 1000 })),
fc.array(fc.integer({ min: 1, max: 1000 })),
(initial, mortalities, sales) => {
const { constrained } = constrainQuantities(
initial,
mortalities,
sales,
)
const current = calculateCurrentQuantity(initial, constrained)
expect(current).toBeGreaterThanOrEqual(0)
expect(current).toBeLessThanOrEqual(initial)
},
),
{ numRuns: 100 },
)
})
})
Linking to Requirements
Annotate tests with requirement links:
typescript
/**
* **Validates: Requirements 3.2, 4.2, 8.2**
*/
it('inventory invariant holds', () => {
// ...
})
When to Use Property Testing
- Mathematical calculations (FCR, mortality rate, profit)
- Invariants (quantity never negative, totals match)
- State transitions (batch status changes)
- Data transformations (currency conversion)
Related Skills
vitest-patterns- Unit testing basicsthree-layer-architecture- Service layer testing
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