Clarify response vs request headers for the setup.js `getContext` function

Describe the bug Whenever the prepare function of src/setup.js returns anything (headers, etc.), the HTTP endpoint causes an Error: Transferred a partial file. This does not happen if the prepare function is not present or does not return anything.

Logs

There is not much to see here. I’ve only tested the endpoints using Insomnia, so no browser logs. (Though, I suspect there wouldn’t be any aside from the aforementioned partial fire error.)

yarn run v1.22.5
warning package.json: No license field
$ svelte-kit dev
(node:83958) ExperimentalWarning: The ESM module loader is experimental.
> Listening on http://localhost:3000

To Reproduce The issue happens after a fresh install of Kit. My setup options were to disable TS in components, and to use SCSS for preprocessing.

I’ve made a repo to show exactly where the issue occurs: @didier/svelte-kit-broken-endpoints. See src/setup.js for an example.

Expected behavior I would expect the endpoint to send a respond, irregardless of whatever the prepare function does and returns.

Stacktraces None, no visible errors occur besides the HTTP client warning me of the Error: Transferred a partial file.

Information about your SvelteKit Installation:

System:
  OS: macOS 11.2.3
  CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Memory: 156.12 MB / 16.00 GB
  Shell: 5.8 - /bin/zsh

Binaries:
  Node: 12.17.0 - ~/.nvm/versions/node/v12.17.0/bin/node
  Yarn: 1.22.5 - /usr/local/bin/yarn
  npm: 6.14.4 - ~/.nvm/versions/node/v12.17.0/bin/npm
  Watchman: 4.9.0 - /usr/local/bin/watchman

Browsers:
  Brave Browser: 89.1.21.77
  Firefox: 83.0
  Firefox Developer Edition: 85.0
  Firefox Nightly: 74.0a1
  Safari: 14.0.3

npmPackages:
  @sveltejs/kit: ^1.0.0-next.55 => 1.0.0-next.55 
  svelte: ^3.29.0 => 3.35.0

Severity This issue is blocking usage of SvelteKit entirely, as HTTP Endpoints are part of the core functionality.

Additional context I’ve had contact with @antony about this. Raising the issue here so other contributors can take a look.

[GrygrFlzr]

Closing since getContext no longer returns a response header, and handle takes in a request object instead, so there’s no longer a header -> header function signature.

[Rich-Harris]

Note that prepare has been replaced by getContext: https://kit.svelte.dev/docs#hooks