mswjs/msw

Support custom WebSocket clients

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#2435 opened on Feb 20, 2025

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 (4 comments) (4 reactions) (0 assignees)TypeScript (13,302 stars) (422 forks)batch import
featurehelp wantedneeds:discussion

Description

Scope

Improves an existing behavior

Compatibility

  • This is a breaking change

Feature description

More context: #2414

Summary

There are cases when tests transitively rely on the WebSocket class that is specification-compliant but is not distributed globally (e.g. the ws package). It would be nice to handle this situation in MSW somehow so it's either supported or made apparent that it won't be.

I believe we should be able to do something like this:

import { ws } from 'msw'
import { WebSocket } from 'ws'

ws.link(url, {
  WebSocket
})

This approach does however have a few gotchas.

Gotchas

It's sensitive to import order. If your tested code gets imported first, it will grab the unpatched version of the client and there's nothing MSW can do about it. Since WebSocket links are created in test suites, perhaps they aren't the best area to support custom WebSocket clients.

This feature would also imply some sort of input validation so users wouldn't provide arbitrary classes hoping that mocking would just work. The WebSocket client must be WHATWG compliant.

Contributor guide

Tech stack
typescript
Domain
testingapi
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Familiarity with WebSocket APIBasic understanding of MSWTypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the existing WebSocket handler in MSW (see #2414 for context). Determine the best way to allow users to pass a custom WebSocket constructor. Consider import order issues and input validation to ensure the provided client is WHATWG compliant. Look at the ws package and its compatibility with the global WebSocket. Possibly create a new link option like ws.link(url, { WebSocket }) and validate it. See discussions in issue comments for maintainer input.