fix(xhr): allow setters for non-existing properties

[kettanaito]

• Fixes TypeError: 'set' on proxy: trap returned falsish for property 'timeout' (Axios, XHR, Happy DOM) msw#1816

Investigation

This issue is caused by the Axios XHR adapter in Happy DOM only. Axios does:

request.timeout = config.timeout

Where request is already a proxy established by Interceptors. It looks up the setProperty trap on the proxy and since timeout doesn't have a special case, it defaults to invoke().

Calling invoke() to set timeout on XMLHttpRequest proxy returns false because property timeout does not exist on target:

interceptors/src/utils/findPropertySource.ts

Lines 9 to 11 in 1f04d10

	if (!(propertyName in target)) {
	return null
	}

The timeout property is not mandatory on XMLHttpRequest, so it's missing from the immediate XHR instance as well as its prototype. We return false from the setter and that causes the following error:

TypeError: 'set' on proxy: trap returned falsish for property 'timeout'

Solution

I think us forbidding setters on unknown properties is a mistake. To fix this, I'm adding a fallback to propertySource to be the immediate target itself is findPropertySource returned null.

[kettanaito]

The actual fix: target is now considered a fallback for propertySource if setting properties that don't exist on target or its prototype.

[kettanaito]

Released: v0.25.9 🎉

This has been released in v0.25.9!

• 📄 Release notes
• 📦 npm package

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

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