property-based-testing

name	property-based-testing
description	fast-check patterns for JS/TS — 6 canonical property categories with worked examples targeting agent-studio utilities (path normalization, safe-json, glob-to-regex, routing logic)
version	1.1.0
verified	true
lastVerifiedAt	2026-03-01
category	Testing
agents	["qa","developer"]
user_invocable	true
invoked_by	both
tools	["Read","Write","Bash"]
tags	["testing","property-based","fast-check","javascript","typescript","fuzzing","invariants"]
best_practices	["State properties as invariants that must hold for ALL inputs","Use shrinkage to get minimal counterexamples on failure","Combine with unit tests — property tests find edge cases, unit tests document examples"]
error_handling	graceful
source	builtin
trust_score	100
provenance_sha	5cad2a35e425e814

Property-Based Testing

fast-check patterns for JavaScript/TypeScript that find edge cases unit tests miss. Includes 6 canonical property categories with worked examples targeting agent-studio's own utilities.

When to Use

• New utility functions (path normalization, parsers, transformers)
• Bug fixes where the fix has a general invariant (not just the specific repro case)
• Security-sensitive string handling (avoid escaping bugs, injection vectors)
• Any function where "this property should hold for ALL inputs" can be stated

The 6 Canonical Property Categories

1. Inverse Operations (Round-trip)

import fc from 'fast-check';
// If you serialize then deserialize, you get back the original
fc.assert(
  fc.property(fc.anything(), value => {
    expect(deserialize(serialize(value))).toEqual(value);
  })
);

2. Idempotency

// Applying a function twice gives same result as once
fc.assert(
  fc.property(fc.string(), str => {
    expect(normalize(normalize(str))).toEqual(normalize(str));
  })
);

3. Commutativity / Order Independence

// Order of inputs shouldn't matter
fc.assert(
  fc.property(fc.array(fc.integer()), arr => {
    expect((arr)).(([...arr].()));
  })
);