property-based-testing

# Property-Based Testing Instead of testing specific examples, define properties that must hold for ALL inputs. The framework generates hundreds of random inputs and finds the smallest failing case. ## When to Use PBT | Use Case | Property | |----------|----------| | Serialization roundtrip | `deserialize(serialize(x)) === x` | | Sort function | Output is ordered AND contains same elements | | Parser | Never crashes on any input | | Encoder/decoder | `decode(encode(x)) === x` | | State machine | Invariants hold after any sequence of operations | | Math/financial | Associativity, commutativity, identity, bounds | | API handler | Never returns 500 on valid input | | Data transformation | Output schema matches specification | ## When NOT to Use PBT - UI rendering tests (use visual regression) - Integration tests with external services (use contract tests) - Tests that need specific business scenarios (use example tests) - Tests where the oracle is as complex as the implementation ## fast-check (JavaScript/TypeScript) ### Setup ```bash npm install --save-dev fast-check ``` ### Basic Property ```typescript import fc from 'fast-check' // Property: sorting is idempotent test('sort is idempotent', () => { fc.assert( fc.property(fc.array(fc.integer()), (arr) => { const sorted = [...arr].sort((a, b) => a - b) const sortedTwice = [...sorted].sort((a, b) => a - b) expect(sorted).toEqual(sortedTwice) }) ) }) // Property: serialization roundtrip tes...