mitt 3.0.1: mitt

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify mitt in pkg:npm/mitt@3.0.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/mitt@3.0.1
Demonstrate these symbols/APIs:
  - mitt
Constraints:
  - executionContext: node
Required runtime conditions:
  - ecosystem: npm
  - language: javascript
  - moduleSystem: cjs
  - packageManager: npm@10.9.8
  - runtime: node@22.23.2

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:69c7ce4ba7500ee255b7a37c004a1fcd0b92175086fdc2dd0b11147097201325","constraints":{"executionContext":"node"},"contract":["mitt() initializes an event emitter instance with an all Map","emitter.on registers event handlers and emitter.emit delivers event payloads to matching listeners","emitter.off removes a specific event handler from the registered handlers","emitter.off without handler removes all registered handlers for the given event type","emitter.on('*', handler) receives event type name and payload for all emitted events","emitter initialized with existing Map shares and preserves pre-populated handlers","emitter.all.clear resets and removes all registered listeners across all event types"],"goal":"verify mitt in pkg:npm/mitt@3.0.1","kind":"HOW","packages":["pkg:npm/mitt@3.0.1"],"schemaVersion":1,"symbols":["mitt"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"esm","os":"linux","osVersionBucket":"24","packageManager":"npm","packageManagerVersion":"10.9.8","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/mitt@3.0.1"],"schemaVersion":1,"subject":"pkg:npm/mitt@3.0.1","symbols":["mitt"],"verifierAdapter":"node-typescript@1"}

{
  "name": "sample-mitt",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-mitt",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "mitt": "3.0.1"
      }
    },
    "node_modules/mitt": {
      "version": "3.0.1",
      "resolved": "https://registry.npmjs.org/mitt/-/mitt-3.0.1.tgz",
      "integrity": "sha512-vKivATfr97l2/QBCYAkXYDbrIWPM2IIKEl7YPhjCvKlG3kE2gm+uBo6nEXK3M5/Ffh/FLpKExzOQ3JJoJGFKBw==",
      "license": "MIT"
    }
  }
}

{
  "name": "sample-mitt",
  "version": "1.0.0",
  "type": "module",
  "private": true,
  "license": "MIT-0",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "mitt": "3.0.1"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify mitt in pkg:npm/mitt@3.0.1",
  "kind": "HOW",
  "packages": [
    "pkg:npm/mitt@3.0.1"
  ],
  "symbols": [
    "mitt"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "packageManager": "npm@10.9.8",
    "runtime": "node@22.23.2"
  }
}

import mitt from 'mitt';

/**
 * Creates and returns a new mitt event emitter instance.
 * @param {Map} [all] Optional Map of event handlers to initialize with.
 * @returns {import('mitt').Emitter} An initialized event emitter.
 */
export function createEmitter(all) {
  return mitt(all);
}

/**
 * Subscribes to an event with a one-time listener that auto-unregisters upon first execution.
 * @param {import('mitt').Emitter} emitter The mitt emitter instance.
 * @param {string|symbol} type Event name to listen for.
 * @param {Function} handler Event handler callback.
 * @returns {Function} Function to remove the once listener manually if needed.
 */
export function once(emitter, type, handler) {
  const wrapped = (event) => {
    emitter.off(type, wrapped);
    handler(event);
  };
  emitter.on(type, wrapped);
  return () => emitter.off(type, wrapped);
}

/**
 * Collects emitted events for a specific event type into an array.
 * @param {import('mitt').Emitter} emitter The mitt emitter instance.
 * @param {string|symbol} type Event name to collect.
 * @returns {{ events: Array<any>, unsubscribe: Function }} Collected events array and unsubscribe function.
 */
export function collectEvents(emitter, type) {
  const events = [];
  const handler = (evt) => {
    events.push(evt);
  };
  emitter.on(type, handler);
  return {
    events,
    unsubscribe: () => emitter.off(type, handler),
  };
}

export { mitt };
export default mitt;

import assert from 'node:assert/strict';
import mitt from 'mitt';
import { createEmitter, once, collectEvents } from '../src/index.mjs';

// 1. mitt() initializes an event emitter instance with an all Map
{
  const emitter = mitt();
  assert.equal(typeof emitter.on, 'function');
  assert.equal(typeof emitter.off, 'function');
  assert.equal(typeof emitter.emit, 'function');
  assert(emitter.all instanceof Map);
  assert.equal(emitter.all.size, 0);

  const helperEmitter = createEmitter();
  assert(helperEmitter.all instanceof Map);
  assert.equal(helperEmitter.all.size, 0);
}

// 2. emitter.on registers event handlers and emitter.emit delivers event payloads to matching listeners
{
  const emitter = mitt();
  const received = [];

  emitter.on('msg', (data) => {
    received.push(data);
  });

  emitter.emit('msg', { text: 'hello' });
  emitter.emit('msg', { text: 'world' });

  assert.deepEqual(received, [{ text: 'hello' }, { text: 'world' }]);
}

// 3. emitter.off removes a specific event handler from the registered handlers
{
  const emitter = mitt();
  const receivedA = [];
  const receivedB = [];

  const handlerA = (val) => receivedA.push(val);
  const handlerB = (val) => receivedB.push(val);

  emitter.on('data', handlerA);
  emitter.on('data', handlerB);

  emitter.emit('data', 1);
  assert.deepEqual(receivedA, [1]);
  assert.deepEqual(receivedB, [1]);

  emitter.off('data', handlerA);

  emitter.emit('data', 2);
  assert.deepEqual(receivedA, [1]);
  assert.deepEqual(receivedB, [1, 2]);
}

// 4. emitter.off without handler removes all registered handlers for the given event type
{
  const emitter = mitt();
  let calls = 0;

  emitter.on('ping', () => calls++);
  emitter.on('ping', () => calls++);

  emitter.emit('ping');
  assert.equal(calls, 2);

  emitter.off('ping');

  emitter.emit('ping');
  assert.equal(calls, 2);
}

// 5. emitter.on('*', handler) receives event type name and payload for all emitted events
{
  const emitter = mitt();
  const wildcardLog = [];

  emitter.on('*', (type, payload) => {
    wildcardLog.push({ type, payload });
  });

  emitter.emit('user:login', { id: 42 });
  emitter.emit('user:logout', { id: 42 });

  assert.deepEqual(wildcardLog, [
    { type: 'user:login', payload: { id: 42 } },
    { type: 'user:logout', payload: { id: 42 } },
  ]);
}

// 6. emitter initialized with existing Map shares and preserves pre-populated handlers
{
  const customMap = new Map();
  const initialLog = [];
  customMap.set('custom', [(data) => initialLog.push(data)]);

  const emitter = mitt(customMap);
  assert.equal(emitter.all, customMap);

  emitter.emit('custom', 'from-custom');
  assert.deepEqual(initialLog, ['from-custom']);
}

// 7. emitter.all.clear resets and removes all registered listeners across all event types
{
  const emitter = mitt();
  let count = 0;

  emitter.on('a', () => count++);
  emitter.on('b', () => count++);
  emitter.on('*', () => count++);

  emitter.emit('a');
  assert.equal(count, 2); // 'a' handler + wildcard handler

  emitter.all.clear();

  emitter.emit('a');
  emitter.emit('b');
  assert.equal(count, 2);
}

// 8. Helper utilities once and collectEvents integrate correctly with mitt lifecycle
{
  const emitter = mitt();
  const onceReceived = [];
  once(emitter, 'single', (data) => onceReceived.push(data));

  emitter.emit('single', 'first');
  emitter.emit('single', 'second');
  assert.deepEqual(onceReceived, ['first']);

  const collector = collectEvents(emitter, 'stream');
  emitter.emit('stream', 100);
  emitter.emit('stream', 200);
  assert.deepEqual(collector.events, [100, 200]);
  collector.unsubscribe();
  emitter.emit('stream', 300);
  assert.deepEqual(collector.events, [100, 200]);
}

console.log('Contract tests passed successfully.');