fetch(new Request()) works with built-in fetch, but not with imported one. Why?

[wonu]

A simple reproduction: https://stackblitz.com/edit/stackblitz-starters-1b5vdt?file=index.js

I tested it also with the following combinations: Node v18.20.3 - Undici v5.28.4, v20.15.0 - v6.13.0 and v22.3.0 - v6.18.2.
Undici versions were matched to the built-in versions of each Node version.

It seems unrelated to the side effects of importing, so I have no idea what's causing this.

[mcollina]

I'm getting this error:

// run `node a.js` in the terminal

const undici = require('./index.js');

// using only one of the two will result an error
const fetch = undici.fetch;
// const Request = undici.Request;

console.log(`Node.js version: ${process.versions.node}`);
console.log(`Built-in Undici version: ${process.versions.undici}`);

fetch(new Request('https://httpbin.org/robots.txt'));

node a.js
Node.js version: 20.15.0
Built-in Undici version: 6.13.0
/Users/matteo/Repositories/undici/index.js:112
      Error.captureStackTrace(err)
            ^

TypeError: Failed to parse URL from [object Request]
    at fetch (/Users/matteo/Repositories/undici/index.js:112:13)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  [cause]: TypeError: Invalid URL
      at new URL (node:internal/url:797:36)
      at new Request (/Users/matteo/Repositories/undici/lib/web/fetch/request.js:115:21)
      at fetch (/Users/matteo/Repositories/undici/lib/web/fetch/index.js:140:21)
      at fetch (/Users/matteo/Repositories/undici/index.js:109:18)
      at Object.<anonymous> (/Users/matteo/Repositories/undici/a.js:12:1)
      at Module._compile (node:internal/modules/cjs/loader:1358:14)
      at Module._extensions..js (node:internal/modules/cjs/loader:1416:10)
      at Module.load (node:internal/modules/cjs/loader:1208:32)
      at Module._load (node:internal/modules/cjs/loader:1024:12)
      at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:174:12) {
    code: 'ERR_INVALID_URL',
    input: '[object Request]'
  }
}

Node.js v20.15.0

Is this what you are seeing?

I think we can do a better error message. TBH, this is expected and I'm not sure we can really change this.

[mcollina]

cc @KhafraDev

[wonu]

Yes, that's right. Thank you for your interest.

I'm not saying that this is a problem, but I was just curious.
I know that Node's fetch uses Undici internally, but does it provide it in a wrapped form instead of using Undici directly?
That's why I thought that the built-in fetch and the imported fetch would be the same if the Undici versions were the same.

I'm sorry if I don't know much about Node.

[mcollina]

Because the two classes are different.

[KhafraDev]

I've gone into detail on other issues about why there isn't a good solution to this but I'm not sure if I could find a link. There isn't much we can do about it.

[wonu]

I got curious about the internal implementation, so I took a closer look.
Thanks for the hint!

// my.js
import { Request as ExternalRequest } from 'undici'
fetch(new ExternalRequest('https://example.com')) // native fetch

// https://github.com/nodejs/undici/blob/main/lib/web/fetch/index.js
function fetch (input, init = undefined) {
  ...
  try {
    requestObject = new Request(input, init)

// https://github.com/nodejs/undici/blob/main/lib/web/fetch/request.js
class Request {
  constructor (input, init = {}) {
    ...
    input = webidl.converters.RequestInfo(input, prefix, 'input')
    ...
    if (typeof input === 'string') {
      ...
      let parsedURL
      try {
        parsedURL = new URL(input, baseUrl) // new URL('[object Request]') throws the error
      } catch (err) {
        throw new TypeError('Failed to parse URL from ' + input, { cause: err })
      }
  ...
}

webidl.converters.RequestInfo = function (V, prefix, argument) {
  if (typeof V === 'string') {
    return webidl.converters.USVString(V, prefix, argument)
  }

  if (V instanceof Request) { // false, because V is an instance of ExternalRequest
    return webidl.converters.Request(V, prefix, argument)
  }

  return webidl.converters.USVString(V, prefix, argument) // returns '[object Request]'
}