Hello! Thanks for making next.js so great.
## Bug
Right now, these types give false confidence. These `key`s are treated as though [a value is defined for _every_ string](https://dev.to/sarioglu/avoiding-unintended-undefined-values-while-using-typescript-record-4igo). However, given an arbitrary request, a particular cookie or query param could be `undefined`.
For example, when building an `/api` endpoint, the code might look like this:
```ts
import type { NextApiRequest, NextApiResponse } from "next"
export default function handler(req: NextApiRequest, res: NextApiResponse) {
// According to the old types, `value` is a string
const value = req.cookies.value
// Type-checking passes but leads to a runtime error when no `value` cookie is provided in the request
// Uncaught TypeError: Cannot read property 'toLowerCase' of undefined
value.toLowerCause()
// ...
}
```
By using `Partial`, TypeScript now knows that these objects don't have values defined for every `key` and accessing a given `key` might resolve to `undefined`.
---
The only obvious error this caused within this repo was on line 333 of the same file. For better or worse, I ended up casting that cookie value to a `string`. There's a series of `if` statements before it that, I guess, are guaranteeing that it's truly a string. Potentially, that stretch could be refactored such that TypeScript _knows_ it's a string.
Also, I tried to follow the contributing guidelines. However, running `yarn types` kicked out a bunch of errors about overwriting files:
```
$ yarn types
yarn run v1.22.10
$ lerna run types --stream
lerna notice cli v4.0.0
lerna info Executing command in 2 packages: "yarn run types"
@next/env: $ tsc index.ts --declaration --emitDeclarationOnly --declarationDir types --esModuleInterop
next: $ tsc --declaration --emitDeclarationOnly --declarationDir dist
next: error TS5055: Cannot write file '/Users/mbrandly/code/next.js/packages/next/dist/build/index.d.ts' because it would overwrite input file.
next: error TS5055: Cannot write file '/Users/mbrandly/code/next.js/packages/next/dist/build/webpack/plugins/build-manifest-plugin.d.ts' because it would overwrite input file.
...
...
...
```
Let me know if there's anything I can improve here! Thanks again.
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