tumblr_form_keys.js

const getResponseText = response => {
  if (response.ok) {
    return response.text();
  } else {
    throw Object.assign(new Error(response.status), {response});
  }
};

const getTumblrFormKey = () => fetch('/neue_web/iframe/new/text')
  .then(getResponseText)
  .then(responseText => {
    const responseDocument = (new DOMParser()).parseFromString(responseText, 'text/html');
    return responseDocument.getElementById('tumblr_form_key').getAttribute('content');
  })
  .catch(console.error);

const getSecureFormKey = () => getTumblrFormKey()
  .then(formKey => fetch('/svc/secure_form_key', {
    method: 'POST',
    headers: { 'x-tumblr-form-key': formKey },
  }))
  .then(response => response.headers.get('x-tumblr-secure-form-key'))
  .catch(console.error);