gistfile1.js

import R from 'ramda';

const createElement = (tag: String): HTMLElement =>
  document.createElement(tag);

const createTextElement = (text: String): HTMLElement =>
  document.createTextNode(text);

const setElementText = (text: String, elem: HTMLElement): HTMLElement => {
  R.compose(
    R.curry(appendTo)(elem),
    createTextElement,
  )(text);
  return elem;
};

const setElementAttrs = (attrs: Object, elem: HTMLElement): HTMLElement => {
  for(let [key, value] of Object.entries(attrs)) {
    elem.setAttribute(key, value);
  }
  return elem;
};

const getBodyElement = (): HTMLElement =>
  document.body

const appendTo = (parent: HTMLElement, child: HTMLElement) =>
  parent.appendChild(child);

const createArrayBuffer = (size: Number) =>
  new Uint8Array(size);

const arrayBufferToBlob = (type: String, arrayBuffer) =>
  new Blob([arrayBuffer], {type});

const blobToURLObject = (blob) =>
  URL.createObjectURL(blob);

document.addEventListener('DOMContentLoaded', function() {

  /* @flow */
  let trolo: Number = 'false';

  R.compose(
    R.curry(appendTo)(getBodyElement()),
    R.curry(setElementAttrs)({
      download: 'image.bmp',
      href: R.compose(
        blobToURLObject,
        R.curry(arrayBufferToBlob)('application/octet-binary'),
      )(createArrayBuffer(10)),
    }),
    R.curry(setElementText)('Download BMP file!'),
    createElement,
  )('a');

});