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EventEmitter3 typed event emitter: working versions (tested 2026-10-15)

Build a typed event emitter with eventemitter3 using an explicit event map, covering on, once, off, listenerCount and removeAllListeners fully offline.

passed4 passed · 0 failed2026-10-15 22:58 UTC
Status
passed
Last passed
2026-10-15 22:58 UTC
Tests
4 / 4
Language
TypeScript
Runtime
node v22.23.3
pinned versions3
eventemitter3@5.0.4@types/node@26.6.5tsx@4.23.15
run it
npm install --no-audit --no-fund --ignore-scripts=false
npm test
test environment
node v22.23.3
linux 6.17.0-1022-azure
x64
runner: github-actions 38820570375

The code

Exactly the files that passed. Your agent can pull them with get_recipe("eventemitter3-typed-emitter") (install), or see them on GitHub.

package.json
{
  "name": "eventemitter3-typed-emitter",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "eventemitter3": "5.0.4"
  },
  "devDependencies": {
    "@types/node": "26.6.5",
    "tsx": "4.23.15"
  }
}
src/emitter.ts
import EventEmitter from "eventemitter3";

export type AppEvents = {
  "user:created": [payload: { id: string; email: string }];
  "order:paid": [payload: { orderId: string; total: number }];
  error: [error: Error];
};

export type TypedEventEmitter = EventEmitter<AppEvents>;

export function createEmitter(): TypedEventEmitter {
  return new EventEmitter<AppEvents>();
}
test/emitter.test.ts
import { test } from "node:test";
import assert from "node:assert/strict";
import { createEmitter, type AppEvents } from "../src/emitter.ts";

test("on delivers typed events and off unsubscribes", () => {
  const emitter = createEmitter();
  const seen: string[] = [];
  const handler = (payload: AppEvents["user:created"][0]) => {
    seen.push(payload.email);
  };

  emitter.on("user:created", handler);
  emitter.emit("user:created", { id: "1", email: "[email protected]" });
  emitter.off("user:created", handler);
  emitter.emit("user:created", { id: "2", email: "[email protected]" });

  assert.deepEqual(seen, ["[email protected]"]);
});

test("once fires at most a single time", () => {
  const emitter = createEmitter();
  let count = 0;
  emitter.once("order:paid", () => {
    count++;
  });

  emitter.emit("order:paid", { orderId: "o1", total: 10 });
  emitter.emit("order:paid", { orderId: "o2", total: 20 });

  assert.equal(count, 1);
});

test("listenerCount reflects registered listeners", () => {
  const emitter = createEmitter();
  assert.equal(emitter.listenerCount("error"), 0);

  const first = () => {};
  const second = () => {};
  emitter.on("error", first);
  emitter.on("error", second);

  assert.equal(emitter.listenerCount("error"), 2);
});

test("removeAllListeners clears one event or the whole emitter", () => {
  const emitter = createEmitter();
  emitter.on("error", () => {});
  emitter.on("order:paid", () => {});

  emitter.removeAllListeners("error");
  assert.equal(emitter.listenerCount("error"), 0);
  assert.equal(emitter.listenerCount("order:paid"), 1);

  emitter.removeAllListeners();
  assert.equal(emitter.listenerCount("order:paid"), 0);
});

Connect to ProvenPin

Add the free MCP server so your agent can pull this recipe and others like it. Same setup as the docs.

terminal
claude mcp add --transport http provenpin https://mcp.provenpin.com/mcp

Any other MCP client: Streamable HTTP at https://mcp.provenpin.com/mcp. No API key, no signup.