How do you post FormData with a duplex (or transform) stream?

[kiyutink]

I was trying to fetch a file from a remote URL and upload it to a different url without saving it to the disk first. That wasn't working and i was getting a RequestError. I did some poking around and noticed that if i use a read stream passed through a simple transform stream, i was getting the same error.
Could someone give me a pointer to what's going wrong? Is it me doing something wrong or the library doesn't support this? If latter, is it something that can be implemented in theory?

// working:

const readStream = fs.createReadStream("file.txt");
const form = new FormData();
form.append("file", readStream);
got.post("https://destination.url", {
  body: form
});


// not working:
const remoteFileStream = got.stream("https://remote-file.url")
const form = new FormData();
form.append("file", remoteFileStream);
got.post("https://destination.url", {
  body: form
});


// not working:

const readStream = fs.createReadStream("file.txt");
const transformStream = new Transform({
  transform(chunk, encoding, callback) {
    callback(null, chunk);
  },
});
const form = new FormData();
form.append("file", readStream.pipe(transformStream));
got.post("https://destination.url", {
  body: form
});

[zzlPetrichor]

I have the same question.
Did you solve it? please.

[kiyutink]

i wasn't able to pipe it directly as I wanted, IIRC this limitation is imposed by the "form-data" library
For me the streaming wasn't critical as i was working with small files, so i just loaded the entire file in memory as buffer and sent it over from memory

[zzlPetrichor]

thank you.
I just solve it.
first, get the response from stream with on event api.
then, replace readStream with response in form-data.
And I find it only works when the got version is over 11.0.0+. (important)

[hctan]

hi @zzlPetrichor , would you be able to share the code snippet how you got it to work? I tried the same way (passing in response) but unable to get it to work. Here's my code:

```
const got = require('got');
const FormData = require('form-data');

const req = got.stream('https://sourceUrl');

req.on('response', async (res) => {
    res.pause();

    const formData = new FormData();
    formData.append('file', res, { filename: 'file1.txt', knownLength: NaN });

    try
    {
        await got.post('https://destinationUrl', {
            body: formData,
        });
    } catch (ex) {
    }
});

I'll get a message saying its response code 408 when it tries to upload the file (got.post()) but I can confirm that the destination server didn't receive the call. It works if I I buffer the stream first and pass the buffer to formData's file.

[sindresorhus]

Posting FormData with streams can be tricky because FormData needs to know the content length upfront. Here's the recommended approach:

import got from 'got';
import FormData from 'form-data';

// Fetch the file as a stream
const sourceStream = got.stream('https://example.com/file.pdf');

// Wait for response event to get headers (including content-length)
sourceStream.on('response', (response) => {
	const form = new FormData();
	
	form.append('file', sourceStream, {
		filename: 'file.pdf',
		contentType: response.headers['content-type'],
		knownLength: parseInt(response.headers['content-length'], 10)
	});
	
	// Upload the form
	got.post('https://upload.example.com', {
		body: form
	});
});

The key is providing knownLength so FormData can set the proper Content-Length header. Without it, the upload may fail or timeout.

If the source doesn't provide a content-length header, you'll need to buffer the stream first, which defeats the puprose of streaming.