snippet of upload code for a signed POST URL with minio

gistfile1.txt

--------------SUMMARY -----------------
  // Uploading a multimedia resource file is a 3-step process:
    //
    // 1. Acquire an "upload ticket"
    // 2. Use the upload ticket to upload the file
    // 3. Acquire a "perma link" to the uploaded file
    //
-------------- SERVER SIDE ----------------------
picture.initiate = function(current, original) {

  var policy = minio.newPostPolicy();
  const filename = random.uuid4() + '.' + current.type;

  policy.setBucket(config.s3.tmpBucket);
  policy.setKey(filename);

  const expires = new Date;
  expires.setSeconds(config.s3.tmpTTL);
  policy.setExpires(expires);
  policy.setContentType('application/octet-stream');  // binary data
  policy.setContentLengthRange(config.s3.uploadSizeMin, config.s3.uploadSizeMax);
  return minio.presignedPostPolicyAsync(policy)
  .spread((url, formData) => {
    current.uploadLink = url;
    current.uploadForm = JSON.stringify(formData);
    current.filename = filename;
    current.state = picture.StateEnum.uploading;
    return current;
  });
};


---------------- CLIENT SIDE ------------------------


function uploadMultimediaResource(file, uploadTicket, onSuccess, onFailure)
{
    console.info(`${objectName}.uploadMultimediaResource(uploadTicket = ${JSON.stringify(uploadTicket)})`);

    // Create the object that will upload the multimedia resource.
    const xhr = new XMLHttpRequest();

    // Specify the HTTP transfer type, URL, and whether the transfer is asynchronous.
    xhr.open('POST', uploadTicket.uploadLink, true);

    // Define the request header.
    const uploadForm = JSON.parse(uploadTicket.uploadForm);
    _.each(uploadForm, (value, key) => {
        console.info(`Setting request header "${key}" to "${value}"`);
        xhr.setRequestHeader(key, value);
    });

    // Define the "onload" event handler.
    xhr.onload = () => {
        if (xhr.status == 200) {
            const responseHeader   = xhr.getRequestHeader();
            const resourceId       = uploadTicket.id;
            const resourceChecksum = 'junk';
            console.info(`${objectName}.uploadMultimediaResource: responseHeader = ${responseHeader}`)
            getMultimediaResourcePermaLink(resourceId, resourceChecksum, onSuccess, onFailure)
        } else {
            const errorMessage = `${objectName}.uploadMultimediaResource(): xhr status code = ${xhr.status}`;
            onFailure(errorMessage);
        }
    };

    // Send the multimedia resource.
    xhr.send(file);

[.....]

and the graphQL in the browser code that interacts with the server that has functional unit tests

function setMultimediaResource(file, onSuccess, onFailure)
{
    console.info(`${objectName}.setMultimediaResource()`);

    // Uploading a multimedia resource file is a 3-step process:
    //
    // 1. Acquire an "upload ticket"
    // 2. Use the upload ticket to upload the file
    // 3. Acquire a "perma link" to the uploaded file
    //
    // Each of these steps is handled by a separate function in this file.
    const mutation = `
        mutation {
          pictureSet (
            picture: {
              description: "",
              type: "${file.type}",
            }
          )
          {
            id,
            description,
            type,
            checksum,
            uploadLink,
            uploadForm,
            state
          }
        }`;
    console.info(`mutation = ${mutation}`);

    httpAgent
        .post(config.persistence.appServerQueryUrl)
        .withCredentials()
        .set('Content-Type', 'application/graphql')
        .set('Accept', 'application/json')
        .send(mutation)
        .then(function(res) {
            console.info(`res = ${JSON.stringify(res)}`);
            if (res.ok && !res.body.errors)
            {  
                uploadMultimediaResource(file, res.body.data.pictureSet, onSuccess, onFailure);
            }   
            else
            {  
                const errorMessage = res.body.errors ? JSON.stringify(res.body.errors) : 'Unknown error';
                console.error(`${objectName}.setMultimediaResource() GraphQL error: ${errorMessage}`)
                onFailure(new Error(errorMessage));
            }
        })
        .catch(function(error) {
            console.error(`Exception in ${objectName}.setMultimediaResource(): ${error} ${error.stack}`); 
            onFailure(error);
        });
}