--- name: bupkis-property-testing-128dfae6b3db description: This skill should be used when the user asks to "write property tests", "add property tests", "create property-based tests", "use @bupkis/property-testing", or mentions "PropertyTestConfig", "fast-check generators", or "property testing for bupkis assertions". Provides guidance for writing property-based tests for bupkis plugin assertions using @bupkis/property-testing and fast-check. --- # Property Testing for Bupkis Plugins This skill covers writing property-based tests for bupkis assertion plugins (like `@bupkis/events` or `@bupkis/sinon`) using `@bupkis/property-testing` and `fast-check`. ## The Spirit of Property Testing **The whole point of property testing is to exercise code with a wide variety of randomly generated inputs.** Every test config should have actual stochastic behavior - if a test just runs the same hardcoded values 50 times, you've missed the plot entirely. ### The Anti-Pattern: `fc.constant(null).chain(...)` This pattern is a red flag that indicates zero randomness: ```typescript // BAD: No randomness at all - just runs the same test 50 times generators: fc.constant(null).chain(() => { const spy = sinon.spy(); spy('hardcoded', 42); return fc.tuple(fc.constant(spy), fc.constant('was called')); }); ``` The only benefit over a regular unit test is exercising alternate phrase variants. That's weak sauce. ### The Fix: Always Generate Something Even when the assertion logic seems simple, find something to randomize: ```typescript // GOOD: Random args, random call counts - actually tests edge cases generators: fc.tuple( fc.integer({ min: 1, max: 5 }), fc.array(fc.oneof(fc.string(), fc.integer(), fc.boolean()), { maxLength: 5 }), ).chain(([callCount, args]) => { const spy = sinon.spy(); for (let i = 0; i < callCount; i++) { spy(...args, i); } return fc.tuple( fc.constant(spy), fc.constantFrom(...extractPhrases(assertions.wasCalledAssertion)), ); }); ``` ### What to Randomize Look for opportunities to vary: - **Arguments/parameters** - strings, numbers, objects, arrays with diverse values - **Counts** - call counts, array lengths, iteration counts - **Object shapes** - context objects, configuration objects - **Error types and messages** - Error, TypeError, RangeError with random messages - **Timing** - delays, timeouts (for async tests) ### Helper Arbitraries Define reusable arbitraries for common patterns: ```typescript // Diverse argument arrays const argsArbitrary = fc.array( fc.oneof( fc.string(), fc.integer(), fc.boolean(), fc.double({ noNaN: true }), fc.constant(null), fc.constant(undefined), ), { maxLength: 5, minLength: 0 }, ); // Random context objects const contextArbitrary = fc.record({ id: fc.oneof(fc.string(), fc.integer()), name: fc.option(fc.string(), { nil: undefined }), value: fc.option(fc.integer(), { nil: undefined }), }); // Random Error instances const errorArbitrary = fc .tuple(fc.string(), fc.constantFrom(Error, TypeError, RangeError)) .map(([msg, ErrorClass]) => new ErrorClass(msg)); ``` ## Overview Property-based testing validates that assertions behave correctly across a wide range of randomly generated inputs. The `@bupkis/property-testing` package provides a test harness that automatically generates four test variants from each configuration: | Variant | Description | | ---------------- | -------------------------------------------------------- | | `valid` | Assertion passes with valid input | | `invalid` | Assertion fails with invalid input | | `validNegated` | Negated assertion passes (e.g., `not to have listeners`) | | `invalidNegated` | Negated assertion fails | ## Setup ### Dependencies Add to `package.json`: ```json { "devDependencies": { "@bupkis/property-testing": "0.15.0", "fast-check": "^4.5.2" } } ``` ### Test File Structure Create `test/property.test.ts`: ```typescript import { createPropertyTestHarness, extractPhrases, getVariants, type PropertyTestConfig, type PropertyTestConfigParameters, } from '@bupkis/property-testing'; import { use } from 'bupkis'; import fc from 'fast-check'; import { describe, it } from 'node:test'; import * as assertions from '../src/assertions.js'; // Create harness with plugin's expect functions const { expect, expectAsync } = use(assertions.myPluginAssertions); const { runVariant } = createPropertyTestHarness({ expect, expectAsync }); // Configure run size ('small' recommended for fast tests) const testConfigDefaults: PropertyTestConfigParameters = { runSize: 'small', // 50 runs; 'medium' = 100; 'large' = 250 } as const; ``` ## PropertyTestConfig Structure Each assertion needs a `PropertyTestConfig` with `valid` and `invalid` variants. **Both variants should have randomness.** ### Generator Tuple Order Generators produce tuples matching assertion signature: `[subject, phrase, ...params]` For `expect(emitter, 'to have listener for', eventName)`: ```typescript generators: fc.string({ minLength: 1 }).chain((eventName) => { const emitter = new EventEmitter(); emitter.on(eventName, () => {}); return fc.tuple( fc.constant(emitter), fc.constantFrom(...extractPhrases(assertions.hasListenerForAssertion)), fc.constant(eventName), ); }); ``` ## The Chain Pattern (Critical) Use `fc.chain()` when generated values must share the same object reference. This is essential for assertions where the subject is configured based on generated values. ### Why Chain Matters Without chaining, each `fc.constant()` creates independent values: ```typescript // WRONG: emitter in tuple is different from configured emitter generators: fc.tuple( fc.constant(new EventEmitter()), // Fresh emitter A fc.constant('to have listeners'), ); // Meanwhile, emitter B was configured elsewhere... ``` With chaining, derived values share the same reference: ```typescript // CORRECT: Same emitter is configured and returned generators: fc.string({ minLength: 1 }).chain((eventName) => { const emitter = new EventEmitter(); emitter.on(eventName, () => {}); // Configure THIS emitter return fc.tuple( fc.constant(emitter), // Return SAME emitter fc.constant('to have listener for'), fc.constant(eventName), ); }), ``` ### Chain Pattern Examples **Single dependency:** ```typescript fc.string({ minLength: 1 }).chain((name) => { const obj = createConfigured(name); return fc.tuple(fc.constant(obj), fc.constant(name)); }); ``` **Multiple dependencies:** ```typescript fc.tuple(fc.string({ minLength: 1 }), fc.integer({ min: 0, max: 10 })).chain( ([name, count]) => { const obj = configure(name, count); return fc.tuple(fc.constant(obj), fc.constant(name), fc.constant(count)); }, ); ``` ## Sync vs Async Assertions ### Sync Assertions Sync assertions use the standard generator pattern. Always include randomness: ```typescript [ assertions.hasListenersAssertion, { valid: { // Random event name - good! generators: fc.string({ minLength: 1 }).chain((eventName) => { const emitter = new EventEmitter(); emitter.on(eventName, () => {}); return fc.tuple( fc.constant(emitter), fc.constantFrom(...extractPhrases(assertions.hasListenersAssertion)), ); }), }, invalid: { // Still use chain to get fresh emitter per run generators: fc.string({ minLength: 1 }).chain(() => { const emitter = new EventEmitter(); // Don't add any listeners - that's the invalid case return fc.tuple( fc.constant(emitter), fc.constantFrom(...extractPhrases(assertions.hasListenersAssertion)), ); }), }, }, ]; ``` ### Async Assertions Async assertions require special handling because they involve triggers and timeouts. **⚠️ Critical: Never test async assertions where "nothing happens"** Do not write tests for async assertions where the trigger is never fired. For example, if the assertion waits for an event that never gets emitted, it will cause timeouts, performance problems, and a world of pain. If you need to test the "invalid" case for an async assertion that waits on something, either: 1. Use a very short timeout (e.g., `within: 50`) and wrap in `'to reject'` (see invalid case example below) 2. Skip the variant entirely if it doesn't make sense to test **Valid cases** use `async: true` flag: ```typescript valid: { async: true, generators: fc.string({ minLength: 1 }).chain((eventName) => { const emitter = new EventEmitter(); return fc.tuple( fc.constant(() => emitter.emit(eventName)), // trigger function fc.constant('to emit from'), fc.constant(emitter), fc.constant(eventName), ); }), } ``` **Invalid cases** use `asyncProperty` with nested rejection: ```typescript invalid: { asyncProperty: () => fc.asyncProperty(fc.string({ minLength: 1 }), async (eventName) => { const emitter = new EventEmitter(); await expectAsync( expectAsync(() => {}, 'to emit from', emitter, eventName, { within: 50, // Short timeout for fast failure }), 'to reject', ); }), } ``` The `asyncProperty` approach is necessary because: 1. Invalid async tests need short timeouts to fail quickly 2. The harness cannot automatically wrap async failures ## Test Harness Loop Run all configs through the harness: ```typescript describe('Property Tests', () => { for (const [assertion, testConfig] of testConfigs) { const { id } = assertion; const { params, variants } = getVariants(testConfig); describe(`Assertion: ${assertion} [${id}]`, () => { for (const [name, variant] of variants) { it(`should pass ${name} checks [${id}]`, async () => { await runVariant( variant, testConfigDefaults, params, name, assertion, ); }); } }); } }); ``` ## Common Patterns ### Using extractPhrases `extractPhrases(assertion)` extracts phrase literals from an assertion definition. **Always prefer `extractPhrases()` over hardcoding phrases** to reduce maintenance burden - if the phrase changes in the assertion definition, tests automatically pick it up. ```typescript // PREFERRED: Uses extractPhrases for maintainability fc.constantFrom(...extractPhrases(assertions.hasListenerForAssertion)); // Works for single or multiple phrase variants // e.g., ['to have listener for'] or ['to have listener for', 'to have a listener for'] ``` ### Compound Assertions (When to Hardcode) Only hardcode phrases for compound assertions like `'to emit from' ... 'with args'` where `extractPhrases()` returns ALL phrases but you need them in separate tuple positions: ```typescript generators: fc.tuple(eventName, args).chain(([eventName, args]) => { const emitter = new EventEmitter(); return fc.tuple( fc.constant(() => emitter.emit(eventName, ...args)), fc.constant('to emit from'), // First phrase only fc.constant(emitter), fc.constant(eventName), fc.constant('with args'), // Second phrase literal fc.constant(args), ); }); ``` ### Testing Invalid Cases with Different Values Generate distinct actual vs expected values: ```typescript fc.tuple( fc.integer({ min: 0, max: 5 }), // actual fc.integer({ min: 0, max: 5 }), // expected ) .filter(([actual, expected]) => actual !== expected) .chain(([actual, expected]) => { const obj = configureWith(actual); return fc.tuple(fc.constant(obj), fc.constant(expected)); }); ``` ### Ensuring Objects Don't Match (Deep Equality / Satisfies) For invalid cases testing `'to satisfy'` or `'deep equal'` semantics, use fast-check's `size` parameter to guarantee mismatches. Generate a smaller object as the subject and a larger one as the expected value—a bigger object will never be a subset of a smaller one: ```typescript fc.tuple( fc.object({ maxDepth: 1, size: 'small' }), // subject (fewer keys) fc.object({ maxDepth: 1, size: 'medium' }), // expected (more keys) ).chain(([subject, expected]) => { return fc.tuple( fc.constant(subject), fc.constantFrom(...extractPhrases(assertions.toSatisfyAssertion)), fc.constant(expected), ); }); ``` This approach avoids flaky tests from random chance matches and doesn't require filtering (which can slow down generation). ### ⚠️ The NaN Trap Remember that `NaN !== NaN` in JavaScript. Strict equality assertions won't behave as expected when `NaN` sneaks into your generated values: ```typescript // This "invalid" case will PASS because NaN !== NaN const subject = NaN; const expected = NaN; expect(subject, 'to be', expected); // Throws! But you wanted it to pass. ``` When using `fc.double()` or `fc.float()`, exclude `NaN` if you're testing strict equality: ```typescript fc.double({ noNaN: true }); // Safe for 'to be' assertions ``` If you actually need to test `NaN` handling, use `Number.isNaN()` or bupkis's `'to be NaN'` assertion instead of strict equality. ## Checklist Before Submitting Before considering property tests complete, verify: 1. **Every config has randomness** - No `fc.constant(null).chain(() => { hardcoded stuff })` 2. **Valid and invalid variants both vary** - Don't just randomize one side 3. **Helper arbitraries are defined** - Reuse common patterns (args, contexts, errors) 4. **Edge cases are possible** - Empty arrays, empty strings, zero, negative numbers 5. **ESLint passes** - Watch for `@typescript-eslint/no-unsafe-return` in forEach callbacks ## Additional Resources ### Reference Files - **`references/generator-patterns.md`** - Detailed fast-check generator patterns - **`references/async-patterns.md`** - Advanced async assertion testing ### Example Files - **`packages/sinon/test/property.test.ts`** - Sync assertions with diverse randomness - **`packages/events/test/property.test.ts`** - Mix of sync and async assertions --- Source attribution: David-Li0406 Original project: https://github.com/David-Li0406/meta-skill-evloving/blob/ca3a335628981df10c36e00cb9850df2c247ab9a/skill-flow/data/skills-refined-36k/skillsmp/bupkis-property-testing/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: David-Li0406 Original project: https://github.com/David-Li0406/meta-skill-evloving/blob/ca3a335628981df10c36e00cb9850df2c247ab9a/skill-flow/data/skills-refined-36k/skillsmp/bupkis-property-testing/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, Frontend Developer, QA / Test Engineer Specialties: Testing Tools: Fast Check, @Bupkis/Property Testing Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: cf866a5ba0c58aadd25f24ff02c2dcc6ee1e89b9bd19b13863f49d6afb894bec