A polyfill for `button type="share"`

sharebutton.js

/* Use <button type="share"> in your HTML.
Include this JavaScript in a <script> element on the same page or in an external script.
The script checks for three ways of sharing:
1. Native support for <button type="share">.
2. Support for the JavaScript Web Share API.
3. A mailto: link.
This will share the current URL and page title.
*/

(function (win, doc) {
const testButton = doc.createElement('button');
testButton.setAttribute('type','share');
if (testButton.type != 'share') {
  win.addEventListener('click', function(ev) {
    ev = ev || win.event;
    let target = ev.target;
    let button = target.closest('button[type="share"]');
    if (button) {
      const title = doc.querySelector('title').innerText;
      const url = win.location.href;
      if (navigator.share) {
        navigator.share({ title: title, url: url });
      } else {
        win.location.href='mailto:?subject='+title+'&body='+url;
      }
    }
  });
}
}(this, this.document));