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

# Using Bun
bun add -d fast-check

# Using npm
npm install -D fast-check

Basic Example

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

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()                       // Hex strings
fc.asciiString()                      // ASCII only
fc.unicodeString()                    // Unicode
fc.emailAddress()                     // Email format

// Arrays and Objects
fc.array(fc.integer())                // Array of integers
fc.array(fc.string(), { minLength: 1, maxLength: 5 })
fc.set(fc.integer())                  // Unique values
fc.record({                           // Objects
  name: fc.string(),
  age: fc.nat(),
})

// Booleans and Constants
fc.boolean()
fc.constant('value')
fc.constantFrom('a', 'b', 'c')        // Pick from options

// Dates
fc.date()
fc.date({ min: new Date('2020-01-01') })

// Complex Types
fc.tuple(fc.string(), fc.integer())   // Fixed-size tuple
fc.oneof(fc.string(), fc.integer())   // Union type
fc.option(fc.string())                // string | null