Fix fetch duplex docs

[Ethan-Arrowood]

I've been toying with the duplex option, and I noticed our docs are a little confusing. The implementation forces me to use 'half' for the value, but it seem like a full duplex operation... unless the 'full' duplex refers to just the request or response individually?

It's a bit confusing.

WDYT @KhafraDev ?

[Ethan-Arrowood]

I wrote up this example:

import http from 'node:http';
import events from 'node:events';
import timers from 'node:timers/promises';

// Create a ReadableStream
const stream = new ReadableStream({
  async start(controller) {
		const chunks = ['Hello', ' ', 'World', '!'];
		for (const chunk of chunks) {
			controller.enqueue(chunk);
			await timers.setTimeout(500);
		}
		controller.close();
  }
});

const server = http.createServer((req, res) => {
	// Gather important information about the request
	const requestInfo = {
		method: req.method,
		url: req.url,
		headers: req.headers,
	};

	// Print the request information
	console.log('Incoming Request:');
	console.log(JSON.stringify(requestInfo, null, 2));

	if (req.method === 'POST') {
		res.writeHead(200, {
			// 'Content-Type': 'text/plain',
			'transfer-encoding': 'chunked'
		})

		req.on('data', (chunk) => {
			console.log('Server Received Chunk:', chunk);
			res.write(chunk);
		})

		req.on('end', ()=> {
			console.log('Server Request Ended');
			res.end();
		})
	} else {
			res.writeHead(200, { 'Content-Type': 'text/plain' });
			res.end('200 OK');
	}
});

server.listen(3000);

await events.once(server, 'listening');

const data = {
  async *[Symbol.asyncIterator]() {
    yield 'hello'
    yield 'world'
  },
}

const response = await fetch('http://localhost:3000', {
  method: 'POST',
  body: stream,
  duplex: 'half'
});

const reader = response.body.getReader();

while (true) {
	const { done, value } = await reader.read();
	if (done) break;
	console.log('Client Received Chunk:', value);
}

server.close();

And it prints out:

Incoming Request:
{
  "method": "POST",
  "url": "/",
  "headers": {
    "host": "localhost:3000",
    "connection": "keep-alive",
    "accept": "*/*",
    "accept-language": "*",
    "sec-fetch-mode": "cors",
    "user-agent": "node",
    "accept-encoding": "gzip, deflate",
    "transfer-encoding": "chunked"
  }
}
Server Received Chunk: <Buffer 48 65 6c 6c 6f>
Client Received Chunk: Uint8Array(5) [ 72, 101, 108, 108, 111 ]
Server Received Chunk: <Buffer 20>
Client Received Chunk: Uint8Array(1) [ 32 ]
Server Received Chunk: <Buffer 57 6f 72 6c 64>
Client Received Chunk: Uint8Array(5) [ 87, 111, 114, 108, 100 ]
Server Received Chunk: <Buffer 21>
Client Received Chunk: Uint8Array(1) [ 33 ]
Server Request Ended

[KhafraDev]

fetch requests are meant to be half-duplex and the duplex option was added to eventually add support for full-duplex requests. Requests in undici are always full-duplex, so our fetch inherited this behavior.

[Ethan-Arrowood]

Cool, sounds good to me!