### 🔖 What's in there?
Because Typescript's `libdom` does not have [`static Response.json()`](https://github.com/microsoft/TypeScript-DOM-lib-generator/issues/1444) (which modern browsers and edge runtimes are supporting), Typescript users can't use easily use it.
This helper fills the gap.
### 🧪 How to test?
It's covered with unit tests
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This PR adds a feature in middleware to add, modify, or delete request
headers. This feature is quite useful to pass data from middleware to
Serverless/Edge API routes.
### New APIs
Adds a new option `request.headers` to the `MiddlewareResponseInit`
parameter in `NextResponse.next()` and `NextResponse.rewrite()`. It's a
[`Header`](https://developer.mozilla.org/en-US/docs/Web/API/Headers)
object holding *all* request headers. Specifically:
```ts
interface ExtraResponseInit extends ResponseInit {
request?: {
headers?: Headers
}
}
```
### Example
```ts
// api/hello.ts
export default (req, res) => {
const valueFromMiddleware = req.headers['x-hello-from-middleware']
return res.send(valueFromMiddleware)
}
// middleware.ts
import { next } from '@vercel/edge'
export default function middleware(request: NextRequest) {
// Clone request headers
const headers = new Headers(request.headers);
// Add a new request header
headers.set('x-hello-from-middleware', 'foo');
// Delete a request header from the client
headers.delete('x-from-client');
return next({
request: {
headers
}
});
}
```
### New middleware headers
- `x-middleware-override-headers`: A comma separated list of *all*
request header names. Headers not listed will be deleted.
- `x-middleware-request-<name>`: A new value for the header `<name>`.
### Related Issues
- Next.js' implementation: https://github.com/vercel/next.js/pull/41380
### 📋 Checklist
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#### Tests
- [ ] The code changed/added as part of this PR has been covered with
tests
- [ ] All tests pass locally with `yarn test-unit`
#### Code Review
- [ ] This PR has a concise title and thorough description useful to a
reviewer
- [ ] Issue from task tracker has a link to this PR
Co-authored-by: Gal Schlezinger <gal@spitfire.co.il>
