Perform worker update in parallel

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 18 or higher

Browsers

No response

Reproduction repository

Any example

Reproduction steps

Have a context that activates MSW.
Have another MSW activation in the same context.

Current behavior

When MSW finds a matching registration and attempts to reuse it, it first updates the worker. That is done to make sure that the existing worker registration is up-to-date. For example, if you are registering a worker that doesn't match the existing worker (i.e. the script has changed due to the MSW version change), the existing worker will be marked as redundant.

msw/src/browser/setupWorker/start/utils/getWorkerInstance.ts

Lines 39 to 40 in 452686d

    
           // When the Service Worker is registered, update it and return the reference. 
        
           return existingRegistration.update().then(() => {

The problem is that .update() takes time. It needs to fetch the worker script, evaluate it byte-by-byte, comparing the old and the new scripts. That's a potential overhead that MSW doesn't have to await.

Expected behavior

Update is scheduled to run in the background. MSW resolves with the existing worker registration immediately.

Released: v2.4.10 🎉

This has been released in v2.4.10!

📄 Release notes
📦 npm package

Make sure to always update to the latest version (npm i msw@latest) to get the newest features and bug fixes.

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	// When the Service Worker is registered, update it and return the reference.
	return existingRegistration.update().then(() => {