property-based-testing

# Property-Based Testing

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

## 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'

// Traditional example-based test
test('reverse twice returns original', () => {
  expect(reverse(reverse([1, 2, 3]))).toEqual([1, 2, 3])
})

// 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!
```

### Built-in Generators

```typescript
import * as fc from 'fast-check'

// Numbers
fc.integer()                          // Any integer
fc.integer({ min: 0, max: 100 })      // Range
fc.nat()                              // Natural numbers (≥ 0)
fc.float()                            // Floating-point
fc.double()                           // Double precision

// Strings
fc.string()                           // Any string
fc.string({ minLength: 1, maxLength: 10 })
fc.hexaString()                       // He