CORS failure results in Uncaught TypeError: Cannot read properties of undefined (reading 'forEach') in recorder.ts

[hugonasciutti]

Hello,

I encountered the following error with posthog-js version 1.166.2:

recorder.ts:688 Uncaught TypeError: Cannot read properties of undefined (reading 'forEach')

This error occurs when my request fails due to a CORS policy restriction. I’ve attached a screenshot for reference.

It seems to be here:

posthog-js/src/entrypoints/recorder.ts

Lines 560 to 563 in 4f73d88

	const responseHeaders: Headers = {}
	res.headers.forEach((value, header) => {
	responseHeaders[header] = value
	})

[pauldambra]

Had you been using previous versions of posthog-js without this?

Absolutely wild that the response would be defined but not have headers. Every time I think that computers have let me down for the last time there's something new 😅

[pauldambra]

What's strange is that we've not had this reported before... are you using other tools that are wrapping fetch?

(the fix is I imagine pretty clear but I'd love to understand the cause in case there's more we need to do!)

[hugonasciutti]

This issue may be related to Supertokens, which I’m using in my project. I noticed that Supertokens overrides the fetch and XMLHttpRequest functions to inject custom headers, so it could be contributing to this bug.

This doesn’t affect my production code; I only noticed it in the console while testing a migration to a newer version of Supertokens.

I’ve used previous versions of posthog-js without encountering this issue, likely because I hadn’t triggered a CORS error like this before.

I thought it would be helpful to open this issue in case others run into the same error log. :)

[pauldambra]

I thought it would be helpful to open this issue in case others run into the same error log. :)

oh, no, absolutely! i'm super pleased you did. my assumption is always that for one person who does open an issue, many people have had the problem and not bothered.

i believe there aren't any recent changes in posthog-js that should contribute here... but that doesn't mean there's nothing for us to do in response