feat: add writeEarlyHints utility (#184)

README.md

@@ -132,6 +132,7 @@ Instead of adding helpers to `req` and `res`, h3 exposes them as composable util
 - `setResponseHeader(event, name, value)` (alias: `setHeader`)
 - `appendResponseHeaders(event, headers)` (alias: `appendHeaders`)
 - `appendResponseHeader(event, name, value)` (alias: `appendHeader`)
+- `writeEarlyHints(event, links, callback)`
 - `createError({ statusCode, statusMessage, data? })`
 - `sendError(res, error, debug?)`
 - `defineHandle(handle)`

src/event/event.ts

@@ -1,5 +1,5 @@
 import type { IncomingMessage as NodeIncomingMessage, ServerResponse as NodeServerResponse } from 'http'
-import type { IncomingMessage, ServerResponse, H3EventContext } from '../types'
+import type { IncomingMessage, ServerResponse, H3EventContext, CompatibilityEvent } from '../types'
 import { MIMES } from '../utils'
 import { H3Response } from './response'
 
@@ -77,3 +77,15 @@ export function isEvent (input: any): input is H3Event {
 export function createEvent (req: NodeIncomingMessage, res: NodeServerResponse): H3Event {
   return new H3Event(req, res)
 }
+
+export function writeEarlyHints (event: CompatibilityEvent, links: string | string[], callback?: () => void) {
+  if (!event.res.socket && !('writeEarlyHints' in event.res)) { return }
+
+  if ('writeEarlyHints' in event.res) {
+    // @ts-expect-error native node 18 implementation
+    return event.res.writeEarlyHints(links, callback)
+  }
+
+  const _links = Array.isArray(links) ? links : [links]
+  event.res.socket!.write(`HTTP/1.1 103 Early Hints\r\nLink: ${_links.join('\r\n')}\r\n\r\n`, 'utf-8', callback)
+}