Fetch in load crashes when using method: "POST"

[MathiasWP]

Describe the bug

Using the fetch method in load says Request with GET/HEAD method cannot have body., even though the method is a "POST" request.

If i use an external endpoint, then the whole application crashes, and if i use a local endpoint then the webpage renders but the error is thrown on the server.

I tried debugging a little bit, and it seems like svelte-kit first throws a GET request, and then afterwards the correct POST request. Note that I am not 100% sure this is the case, it's just a hypothesis after adding logs around in node/polyfills.js.

I wanted to try and solve this issue myself because this is a critical bug for our application, but i sadly don't have the time today. I will help as much as i can if anyone will help solving this issue asap.

Reproduction

https://github.com/MathiasWP/sveltekit-load-external-fetch-post-error

Logs

No response

System Info

System:
    OS: macOS 11.4
    CPU: (8) x64 Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
    Memory: 28.71 MB / 8.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.15.1 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 8.13.1 - /usr/local/bin/npm
  Browsers:
    Brave Browser: 103.1.40.107
    Chrome: 103.0.5060.114
    Firefox: 100.0
    Safari: 14.1.1
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.54 
    @sveltejs/kit: 1.0.0-next.360 => 1.0.0-next.360 
    svelte: ^3.44.0 => 3.48.0 
    vite: ^2.9.13 => 2.9.13

Severity

blocking all usage of SvelteKit

Additional Information

No response

[david-plugge]

#5383

[Conduitry]

Yup, closing as duplicate of #5383.