property-testing

name	property-testing
description	fast-check property-based testing for pure functions. Use when writing property-based tests or testing pure functions. Triggers: "property", "fast-check", "fc.assert", "invariant", "pure function".
category	web-ui

Skill: property-testing

What Is Property-Based Testing

Instead of testing specific examples, test properties that should hold for all inputs.

import fc from 'fast-check';

test('escapeHtml never contains raw < or >', () => {
  fc.assert(
    fc.property(fc.string(), (s) => {
      const escaped = escapeHtml(s);
      expect(escaped).not.toMatch(/</);
      expect(escaped).not.toMatch(/>/);
    })
  );
});

When to Use

• Pure functions (formatCount, escapeHtml, etc.)
• Data transformations
• Parsers/validators
• Any function where "for all inputs" matters

Rules

• Test properties, not examples
• Use fc.assert with fc.property
• Filter inputs when needed: .filter(s => s.length > 0)
• Run 100+ iterations in CI

Commands

# Run property tests
bun vitest run tests/property/