There is bug in this code.
The following part is buggy,

sections.push({
        'headerStart': arraybuf.byteLength - boundary.length - 2 // 2 hyphens at end
      });

You are removing boundary length two times for last section. You already remove boundary when define section.end. So, fix is following

sections.push({
        'headerStart': arraybuf.byteLength - 2 // 2 hyphens at end
      });

Modern browsers can parse multipart/form-data natively. Example:

const payload =
`------WebKitFormBoundaryU5rJUDxGnj15hIGW\r
Content-Disposition: form-data; name="field1"\r
\r
Hello
World,
This is me\r
------WebKitFormBoundaryU5rJUDxGnj15hIGW--`

const boundary = payload.slice(2, payload.indexOf('\r\n'))
new Response(payload, {
  headers: {
    'Content-Type': `multipart/form-data; boundary=${boundary}`
  }
})
  .formData()
  .then(formData => {
    console.log([...formData]) // [['field1', 'Hello\nWorld,\nThis is me']]
  })

The \r inside payload are necessary, because the line breaks must be \r\n, except the values themselves. If you have a properly formed multipart/form-data blob, you don't need to add \r.

If you want to parse an HTTP response, you can use the fetch response directly:

fetch('/formdata-response')
  .then(response => response.formData())
  .then(formData => console.log([...formData]))

Modern browsers can parse multipart/form-data natively. Example:

Thank you! We've come a long way since 2017

@Finesse entries() is not necessary, FormData spreads to an array of arrays (entries) [...fd].

@guest271314 You are right, I've amended my code snippet

Very helpful snippet. If you have access to fetch() it should be possible to use text() to get the raw multipart/form-data content with \r\n included

{
  var formdata = new FormData();
  var dirname = "web-directory";
  formdata.append(dirname, new Blob(["123"], {
    type: "text/plain"
  }), `${dirname}/file.txt`);
  formdata.append(dirname, new Blob(["src"], {
    type: "text/plain"
  }), `${dirname}/src/file.txt`);
  var body = await new Response(formdata).text();
  console.log(body);
  const boundary = body.slice(2, body.indexOf('\r\n'))
  console.log(boundary)
  new Response(body, {
      headers: {
        'Content-Type': `multipart/form-data; boundary=${boundary}`
      }
    })
    .formData()
    .then(formData => {
      console.log([...formData])
    })
}

@Finesse Another way to do this when payload is a TypedArray (or ArrayBuffer)

let ab = new Uint8Array(await response.clone().arrayBuffer());

let boundary = ab.subarray(2, ab.indexOf(13) + 1);

let archive = await new Response(ab, {
    headers: {
      "Content-Type": `multipart/form-data; boundary=${new TextDecoder().decode(boundary)}`,
    },
  })
  .formData()
  .then((data) => {
    console.log([...data]);
    return data;
  }).catch((e) => {
    console.warn(e);
  });