Sending a file using fetch() without first buffering it in memory

[gabrielschulhof]

The following was possible with Node.js 22:

const { Readable } = require('node:stream');
const fs = require('node:fs');

const accessToken = process.env.ACCESS_TOKEN;
const url = process.env.URL;
const filename = process.env.FILENAME;

class MyFile {
  constructor(options) {
    Object.assign(this, options);
    this[Symbol.toStringTag] = 'File';
  }
}

;(async () => {
  const stream = Readable.toWeb(fs.createReadStream(filename));
  const form = new FormData();

  form.set('file', new MyFile({
    stream: () => stream,
    name: filename,
    type: 'application/octet-stream'
  }));

  const result = await fetch(url, {
    method: 'PUT',
    body: form,
    headers: {
      authorization: `Bearer ${accessToken}`
    }
  });

  console.log(`ok: ${result.ok}`)

  console.log(JSON.stringify(await result.json(), null, 2));
})()

IOW, one could supply a pseudo-File instance to fetch(), which would accept it because of a loose isBlobLike() type check in its implementation, and would then stream it into a part of a multipart form request.

In Node.js 24, this type check is now much stricter in that the instance must be an instance of Blob, which causes the above code to simply upload the string [object File] instead of the file itself.

Thus, in Node.js 24, is there a way to send a file using fetch() without first buffering it in memory?

[gabrielschulhof]

Solution: Pass the FormData to FormDataEncoder, and add the content-type: form/multipart; boundary=${formDataRecorder.boundary} header.

It'd be nice to do this natively with Node.js though.

[targos]

Maybe with fs.openAsBlob?

[gabrielschulhof]

@targos thanks! Unfortunately my example was only an illustration. In the actual code, I receive a Readable from an external library and I need to convert that to a blob without draining it first. In my example I used fs.createReadStream() as a stand-in for the Readable I receive. Sorry I forgot to make that clear!