spirobel•5mo ago

How to feed a ReadableStream / async Generator into a Response object?

`` Bun.serve({
    async fetch(req) {
        return new Response(
            async function* stream() {
                         // ~~~~~~~^?
                yield "Hello, ";
                yield Buffer.from("world!");
            },
        );
    },
});

it is possible to feed an async generator directly into the fetch Response with Bun. With deno this does not work. I also tried to turn it into a ReadableStream with ReadableStream.from(), but no success so far. Is this the correct way to produce a ReadableStream from an async generator, or do I have to do this differently? thanks!

5 Replies

spirobel•5mo ago

another try using the node:stream Readable. works in bun does not work in deno

spirobel•5mo ago

https://docs.deno.com/deploy/api/runtime-response

HTTP Response | Deno Docs

The Response

zamfofex•5mo ago

If you want to be able to mix and match strings and buffers, you need a preprocessor function like this:

// mix strings and buffers

Deno.serve(request => new Response(ReadableStream.from(preprocess(request))))

let encoder = new TextEncoder()

async function * preprocess(request)
{
    for await (let chunk of makeStream(request))
    {
        if (typeof chunk === "string") chunk = encoder.encode(chunk)
        yield chunk
    }
}

async function * makeStream(request)
{
    yield "Hello, "
    yield encoder.encode("world!")
}

// mix strings and buffers

Deno.serve(request => new Response(ReadableStream.from(preprocess(request))))

let encoder = new TextEncoder()

async function * preprocess(request)
{
    for await (let chunk of makeStream(request))
    {
        if (typeof chunk === "string") chunk = encoder.encode(chunk)
        yield chunk
    }
}

async function * makeStream(request)
{
    yield "Hello, "
    yield encoder.encode("world!")
}

Otherwise, you can get away with only buffers or only strings more easily:

// only buffers
Deno.serve(request => new Response(ReadableStream.from(makeStream(request))))

let encoder = new TextEncoder()

async function * makeStream(request)
{
    yield encoder.encode("Hello, ")
    yield encoder.encode("world!")
}

// only buffers
Deno.serve(request => new Response(ReadableStream.from(makeStream(request))))

let encoder = new TextEncoder()

async function * makeStream(request)
{
    yield encoder.encode("Hello, ")
    yield encoder.encode("world!")
}

// only strings
Deno.serve(request => new Response(ReadableStream.from(makeStream(request)).pipeThrough(new TextEncoderStream())))

async function * makeStream(request)
{
    yield "Hello, "
    yield "world!"
}

// only strings
Deno.serve(request => new Response(ReadableStream.from(makeStream(request)).pipeThrough(new TextEncoderStream())))

async function * makeStream(request)
{
    yield "Hello, "
    yield "world!"
}

spirobel•5mo ago

thanks a lot for your reply! I didn't know yielding buffers or strings mattered. this is great info! is there something special I need to do to flush the buffers after every yield? I tried this:

 javascript
Deno.serve(
  (request) =>
    new Response(
      ReadableStream.from(makeStream(request)).pipeThrough(
        new TextEncoderStream()
      )
    )
);
const wait = (n: number) => new Promise((resolve) => setTimeout(resolve, n));

async function* makeStream(request) {
  yield "Hello,";
  console.log("start wait ");
  await wait(5000);
  yield "world!";
}

 javascript
Deno.serve(
  (request) =>
    new Response(
      ReadableStream.from(makeStream(request)).pipeThrough(
        new TextEncoderStream()
      )
    )
);
const wait = (n: number) => new Promise((resolve) => setTimeout(resolve, n));

async function* makeStream(request) {
  yield "Hello,";
  console.log("start wait ");
  await wait(5000);
  yield "world!";
}

it waited 5 seconds and then sent the whole Hello world. I thought it would send Hello, then wait for 5 seconds and then send the world! 😀 (while sending the Hello instantly) EDIT: with { headers: { "content-type": "text/html; charset=utf-8" } } it works