Presigned POST

Form.js

const s3 = new AWS.S3({region: 'us-west-2'});

s3.createPresignedPost({
  Bucket: 'bucket',
  Fields: {
    key: 'user_files/${filename}',
  },
  Conditions: [
    ['starts-with', '$key', '']
  ]
}, function (err, data) {
  const form = document.createElement('form');
  form.setAttribute('action', data.url);
  form.setAttribute('method', 'POST');
  form.setAttribute('enctype', 'multipart/form-data');
  Object.keys(data.fields).forEach(function(key) {
    const input = document.createElement('input');
    input.setAttribute('type', 'hidden');
    input.setAttribute('name', key);
    input.setAttribute('value', data.fields[key]);
    form.appendChild(input);
  });

  const fileInput = document.createElement('input');
  fileInput.setAttribute('type', 'file');
  fileInput.setAttribute('name', 'file');
  form.appendChild(fileInput);

  const submitButton = document.createElement('input');
  submitButton.setAttribute('type', 'submit');
  submitButton.setAttribute('name', 'submit');
  submitButton.setAttribute('value', 'submit');
  form.appendChild(submitButton);

  document.getElementById('form-target').appendChild(form);
});

FormData.js

s3.createPresignedPost({
    Expires: 60 * 60 * 24,
    Bucket: 'my-bucket',
    Fields: {
        acl: 'public-read',
        'x-amz-server-side-encryption': 'AES256',
        'x-amz-meta-uuid': 'UUID',
    },
    Conditions: [
        ['starts-with', '$key', 'user/'],
        ["starts-with", "$Content-Type", "image/"],
        ['starts-with', '$x-amz-meta-tag', ''],
    ],
}, (err, data) => {
    const form = new FormData();
    const {fields, url} = data;
    for (let field of Object.keys(fields)) {
        form.append(field, fields[field]);
    }

    form.append('key', 'user/${filename}');
    form.append('Content-Type', 'image/jpg');
    form.append('x-amz-meta-tag', '');

    let xhr = new XMLHttpRequest();
    xhr.withCredentials = true;

    xhr.addEventListener("readystatechange", function () {
        if (this.readyState === 4) {
            console.log(this.responseText);
        }
    });

    xhr.open("POST", "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX");
    xhr.send(data);
});