Not able to set content-length on outgoing Next API responses

[heydev]

I'm not sure if I have a true issue here or not, as I don't know if I'm just doing something wrong, or if there is a real issue with Next.

I have a simple requirement to provide a % complete progress bar on a resource download. Everything works except the progress never moves from zero because content-length is not being set on responses from my NextJS API route, so Axios cannot compute the total value in the onDownloadProgress handler input.

When I try to set the content-length manually in the response, it is removed. I have confirmed I can set other header values.

How can I enable NextJS API routes to return valid content-length values? Thanks!

[thibautsabot]

Hello!

It seems that Next.js removes this header from the API routes https://github.com/vercel/next.js/blob/canary/packages/next/server/api-utils/node.ts#L370 for 204 and 304 status.

Are these the statuses you are returning? It might be the case because of a cache.
If not, the Content-Length should be automatically filled with the length of the body/payload.

[heydev]

Okay, so was able to do some troubleshooting. Based on some things I read on the web, it looked like certain scenarios disabled Content-Length when encoding (gzip, br, etc) was used. My axios call in the client side code is populating those values by default (seems sensible), and whenever the Accept-Encoding header is sent, the response comes back without Content-Length.

When I send a hand-rolled request to my API without Accept-Encoding, the response includes Content-Length.

[heydev]

So, understandably, you can't set Accept-Encoding in the browser. Chrome just won't let you do it. I was able to "make it work" by disabling compression across the whole NextJS app, which feels bad. With compress: false in my next.config.js, I now get Content-Length values set in my API responses.

I guess I'm still uncertain why compression enabled would cause content-length values to go away?

[heydev]

This SO answer seems to describe exactly what I'm seeing (though Express, and not NextJS). When compression is enabled, I do indeed get a Transfer-Encoding: chunked. Maybe this is ultimately an underlying NodeJS thing, and not a NextJS thing? Though even there, it looks like to disable this in NodeJS, you only need to include the Content-Length, which I did, but is being removed somewhere down the line.

[thibautsabot]

Thank you for sharing your findings.
Maybe you can disable compression for the specific endpoint from your API?

I did not have the issue with a simple fetch to https://jsonplaceholder.typicode.com/

[heydev]

compress: false did not seem to make an impact on the API config, and the documentation is a little sparse on what all the available options are there.