mitt 3.0.1: emitter.on

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 emitter.on 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:
  - emitter.on
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:sha256:359911096b8da0a7a3a184875fb14e57c808b02dfa67a824c6bf5e43507d10b6","constraints":{"executionContext":"node"},"contract":["emitter.on registers a callback for a specific event type and invokes it with the payload when emitted","emitter.on preserves listener registration order when multiple handlers are registered for the same event","emitter.on supports registering wildcard listeners with asterisk to receive all event types and payloads","emitter.on supports symbol event types for private or unique event channels","emitter.on registers duplicate handler references so each registration is invoked on emit","emitter.on registered handlers can be removed using emitter.off"],"goal":"verify emitter.on in pkg:npm/mitt@3.0.1","kind":"HOW","packages":["pkg:npm/mitt@3.0.1"],"schemaVersion":1,"symbols":["emitter.on"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"cjs","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":["emitter.on"],"verifierAdapter":"node-typescript@1"}

{
  "name": "sample-mitt-emitter-on",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-mitt-emitter-on",
      "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-emitter-on",
  "version": "1.0.0",
  "private": true,
  "description": "Clean-room code sample for mitt@3.0.1 verifying emitter.on",
  "main": "src/index.js",
  "license": "MIT-0",
  "dependencies": {
    "mitt": "3.0.1"
  }
}

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

'use strict';

const mitt = require('mitt');

/**
 * Creates an event emitter instance.
 *
 * @param {Map} [all] - Optional map of event types to handler arrays.
 * @returns {object} Mitt emitter instance.
 */
function createEmitter(all) {
  return mitt(all);
}

/**
 * Registers an event listener on the given emitter.
 *
 * @param {object} emitter - The emitter instance.
 * @param {string|symbol} type - Event type name or symbol, or '*' for wildcard.
 * @param {Function} handler - Event handler function.
 */
function registerListener(emitter, type, handler) {
  emitter.on(type, handler);
}

/**
 * Registers a wildcard listener that intercepts all emitted events.
 *
 * @param {object} emitter - The emitter instance.
 * @param {Function} handler - Wildcard handler receiving (type, event).
 */
function registerWildcardListener(emitter, handler) {
  emitter.on('*', handler);
}

module.exports = {
  mitt,
  createEmitter,
  registerListener,
  registerWildcardListener
};

import assert from 'node:assert/strict';
import { createRequire } from 'node:module';

const require = createRequire(import.meta.url);
const { mitt, createEmitter, registerListener, registerWildcardListener } = require('../src/index.js');

async function testAll() {
  // 1. Export verification
  assert.equal(typeof mitt, 'function', 'mitt default export must be a function');
  assert.equal(typeof createEmitter, 'function', 'createEmitter helper must be a function');
  assert.equal(typeof registerListener, 'function', 'registerListener helper must be a function');
  assert.equal(typeof registerWildcardListener, 'function', 'registerWildcardListener helper must be a function');

  // 2. emitter.on registers a callback for a specific event type and invokes it with the payload when emitted
  {
    const emitter = createEmitter();
    const received = [];
    registerListener(emitter, 'user:login', (data) => {
      received.push(data);
    });

    emitter.emit('user:login', { id: 101, username: 'tester' });
    emitter.emit('other:event', { id: 999 });

    assert.equal(received.length, 1, 'handler should be called exactly once');
    assert.deepEqual(received[0], { id: 101, username: 'tester' }, 'handler should receive the emitted payload');
  }

  // 3. emitter.on preserves listener registration order when multiple handlers are registered for the same event
  {
    const emitter = createEmitter();
    const order = [];

    emitter.on('task', (val) => {
      order.push(`first-${val}`);
    });
    emitter.on('task', (val) => {
      order.push(`second-${val}`);
    });
    emitter.on('task', (val) => {
      order.push(`third-${val}`);
    });

    emitter.emit('task', 'run');
    assert.deepEqual(order, ['first-run', 'second-run', 'third-run'], 'handlers should execute in registration order');
  }

  // 4. emitter.on supports registering wildcard listeners with asterisk to receive all event types and payloads
  {
    const emitter = createEmitter();
    const captured = [];

    registerWildcardListener(emitter, (type, payload) => {
      captured.push({ type, payload });
    });

    emitter.emit('event:alpha', 100);
    emitter.emit('event:beta', { status: 'ok' });

    assert.equal(captured.length, 2, 'wildcard handler should capture all events');
    assert.deepEqual(
      captured,
      [
        { type: 'event:alpha', payload: 100 },
        { type: 'event:beta', payload: { status: 'ok' } }
      ],
      'wildcard handler should receive event type and payload'
    );
  }

  // 5. emitter.on supports symbol event types for private or unique event channels
  {
    const emitter = createEmitter();
    const symbolKey = Symbol('customEvent');
    let symbolPayload = null;

    emitter.on(symbolKey, (payload) => {
      symbolPayload = payload;
    });

    emitter.emit(symbolKey, 'symbol-payload-value');
    assert.equal(symbolPayload, 'symbol-payload-value', 'symbol event handler should receive payload');
  }

  // 6. emitter.on registers duplicate handler references so each registration is invoked on emit
  {
    const emitter = createEmitter();
    let counter = 0;
    const increment = () => {
      counter++;
    };

    emitter.on('tick', increment);
    emitter.on('tick', increment);

    emitter.emit('tick', null);
    assert.equal(counter, 2, 'duplicate registrations of same handler should each execute');
  }

  // 7. emitter.on registered handlers can be removed using emitter.off
  {
    const emitter = createEmitter();
    const events = [];
    const handler = (val) => {
      events.push(val);
    };

    emitter.on('item', handler);
    emitter.emit('item', 1);

    emitter.off('item', handler);
    emitter.emit('item', 2);

    assert.deepEqual(events, [1], 'handler should not receive events after off()');
  }

  console.log('All mitt emitter.on contract assertions passed.');
}

testAll().catch((err) => {
  console.error('Contract test failure:', err);
  process.exit(1);
});