How to get response size in middleware

[itssimon]

I want to implement a middleware that captures the size of the response. Most frameworks set the Content-Length header, but Hono does not seem to. How could one achieve this?

[maou-shonen]

This is just a small example. In reality, you might need to consider encoding and streaming, etc.

import { Hono } from "hono"
import type { MiddlewareHandler } from "hono"

const contentSize: MiddlewareHandler = async (c, next) => {
  await next()
  const res = c.res.clone()
  const body = await res.text()
  const contentLength = Buffer.byteLength(body, "utf8")
  c.res.headers.set("Content-Length", contentLength.toString())
}

const app = new Hono().use(contentSize).get("/:msg", (c) => {
  const msg = c.req.param("msg")
  return c.text(msg)
})

const res = await app.request("/hello")
console.log(res.headers.get("Content-Length")) // output: 5

[itssimon]

For future reference, I ended up implementing it like this:

const teeResponse = async (
  response: Response,
): Promise<[Response, Response]> => {
  if (!response.body) {
    return [response, response];
  }
  const [stream1, stream2] = response.body.tee();
  const newResponse1 = new Response(stream1, {
    status: response.status,
    statusText: response.statusText,
    headers: response.headers,
  });
  const newResponse2 = new Response(stream2, {
    status: response.status,
    statusText: response.statusText,
    headers: response.headers,
  });
  return [newResponse1, newResponse2];
};

const measureResponseSize = async (
  response: Response,
): Promise<[number, Response]> => {
  const [newResponse1, newResponse2] = await teeResponse(response);
  let size = 0;
  if (newResponse2.body) {
    let done = false;
    const reader = newResponse2.body.getReader();
    while (!done) {
      const result = await reader.read();
      done = result.done;
      if (!done && result.value) {
        size += result.value.byteLength;
      }
    }
  }
  return [size, newResponse1];
};