This article shows how to upload a file to Amazon S3 or minio using a form. I want to use the pre-signed POST URL policy method, since it appears to be the most secure.

However, I wish to upload a file programmatically to minio or S3 using a browser. In other words, I want to use javascript, not a form.

I'm a node user and familiar with superagent, which is very simple to use using the .field() and .attach() operation to set the form fields and to upload the file.

This works in node:

  let cdnAgent = superagent;
  let req = cdnAgent.post(r.data.pictureSet.uploadLink);
  _.each(uploadForm, function(value, key) {
    req.field(key, value);
  });
  // upload file via the created signed policy
  return req
  .set('Content-Type', 'application/octet-stream')
  .attach('file', 'test/data/test.jpg');
}).then(r => {

However, this does not work in a browser, .attach() is not supported, and when I worked around that, I ran into CORS issues since my javascript domain is a separate domain from my minio or S3 domain.

To answer this question I'd like to see a complete example using XMLHttpRequest, Axios, superagent, or Fetch, that works across domains.

I note there are some similar questions to this and some answers, but they all appear to be very outdated and technology has moved on. So I don't think this is a duplicate question.