fast-check Overview fast-check is a property-based testing framework — instead of writing specific test cases, you describe properties that should always hold, and fast-check generates thousands of random inputs to try to break them. When it finds a failing input, it automatically shrinks it to the minimal reproducing case. Catches edge cases you'd never think to test: empty strings, negative numbers, Unicode, massive arrays, boundary values. When to Use - Functions with many possible inputs (parsers, validators, serializers) - Finding edge cases in business logic (pricing, permissions, date…