Duplicate log output and MaxListenersExceededWarning warning (regression in 0.47.1)

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn’t contain the answer to my issue

Environment check

• I’m using the latest msw version
• I’m using Node.js version 14 or higher

Browsers

Chromium (Chrome, Brave, etc.), Firefox

Reproduction repository

https://github.com/kleinfreund/kuma-gui/tree/feat/rework-dataplanes-view

Reproduction steps

1. In package.json, change the msw dependency from 0.47.0 to ^0.47.3
2. Run yarn install
3. Run yarn run dev
4. Browse around the UI and observe the console log entries

Current behavior

Many redundant log entries appear for requests that are only made once (and handled once). Example:

RestHandler.ts:183 [MSW] 09:08:29 GET http://localhost:5681/ (200 OK)

The following warning appears:

events.js:46 MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 response:mocked listeners added. Use emitter.setMaxListeners() to increase limit
    at _addListener (http://localhost:8080/node_modules/.vite/deps/msw.js?v=9b1b86e7:597:19)
    at StrictEventEmitter2.addListener (http://localhost:8080/node_modules/.vite/deps/msw.js?v=9b1b86e7:608:14)
    at StrictEventEmitter2.once (http://localhost:8080/node_modules/.vite/deps/msw.js?v=9b1b86e7:868:36)
    at Object.onMockedResponse (http://localhost:8080/node_modules/.vite/deps/msw.js?v=9b1b86e7:22101:34)
ProcessEmitWarning		@	events.js:46
_addListener			@	events.js:219
addListener			@	events.js:227
StrictEventEmitter2.once	@	StrictEventEmitter.js:34
onMockedResponse		@	createRequestListener.ts:76
await in onMockedResponse (async)		
handleRequest			@	handleRequest.ts:127
await in handleRequest (async)		
(anonymous)			@	createRequestListener.ts:34
(anonymous)			@	setupWorker.ts:93

Expected behavior

The logs aren’t unnecessarily redundant and there is no max listeners exceeding warning.

Notes

• Likely cause as the only change between 0.47.0 and 0.47.1: https://github.com/mswjs/msw/pull/1392
• Initially reported in https://github.com/mswjs/msw/issues/1343#issuecomment-1246566445
• Code that reproduces the issue does not import msw/node

[kettanaito]

@snaka, that must have been the root cause. We’ve just merged and released the fix to strict-event-emitter. Would anyone be interested in bumping that dependency in MSW in a pull request?

On a related note, I’ve noticed that https://github.com/open-draft/strict-event-emitter is heavily under-tested. If you’d like to contribute to a package we use extensively in MSW and beyond, writing tests for strict-event-emitter is a fantastic place to start!

[kleinfreund]

@snaka That looks promising considering that https://github.com/mswjs/msw/pull/1392 introduced calls to context.emitter.once.