Simplest way to generate and download a file using Javascript

download.js

function download(filename, text){
  // MIME types: https://www.freeformatter.com/mime-types-list.html#mime-types-list
  var blob = new Blob([text], {type: "text/plain"});
  var url = window.URL.createObjectURL(blob);
  var a = document.createElement("a");
  a.href = url;
  a.download = filename;
  a.click();
}

download("text.txt", "file contents here...");

google-book-image-downloader.js

var saved = {};
setInterval(() => savePages(), 200);

async function savePages() {
  const srcs = Array.from(document.querySelectorAll('img[src^="https://books.google.com/books/content"][style]')).map(el => el.src);
  for (let i = 0; i < srcs.length; i++) {
    const src = srcs[i];
    const page = getParam(src, 'pg');
    if (!saved[page]) {
      saved[page] = true;
      const blob = await fetch(src).then(res => res.blob());
      download(blob, page + '.png');
    }
  }
}

function getParam(url, param){
  url = new URL(url);
  return new URLSearchParams(url.search).get(param);
}

function download(blob, filename) {
  var url = window.URL.createObjectURL(blob);
  var a = document.createElement("a");
  a.href = url;
  a.download = filename;
  a.click();
}

image-downloader.js

(async () => {
  // https://utah.pbslearningmedia.org/resource/ae339529-1ab8-4cb2-bdbe-75b94287d6c9/us-states-shapes-and-names-clipart/
  
  const state = document.querySelector('.item-description').innerText.match(/state of (.+) as/)[1];
  const img = document.querySelector('.row.gallery-item-wrapper .poster-image');
  const url = img.src.split('.resize')[0];

  const blob = await fetch(url).then(res => res.blob());
  download(blob, state + '.png');

  function download(blob, filename) {
    var url = window.URL.createObjectURL(blob);
    var a = document.createElement("a");
    a.href = url;
    a.download = filename;
    a.click();
  }

  document.querySelector('.preview-button.preview-right').click()

})();