Tampermonkey - GM_setClipboard.js

var copy = function(opts) {
  if (typeof opts === 'string') {
    opts = {
      content: opts,
      mimetype: 'text/plain'
    };
  }
  var frame = document.createElement("iframe");
  document.body.appendChild(frame);
  try {
    frame.contentDocument.designMode = "on";
    /*
        # if html is opts.type
        #   frame.setAttribute "sandbox", "allow-same-origin"
        #   frame.contentDocument.documentElement.innerHTML = opts.content
        #   frame.contentDocument.execCommand "selectAll", not 1, null
        # else
        */
    frame.contentDocument.oncopy = function(b) {
      b.clipboardData.setData(opts.mimetype || "text/plain", opts.content);
      return b.preventDefault();
    };
    frame.contentDocument.execCommand("copy", !1, null);
    frame.contentDocument.designMode = "off";
  } catch (err) {
    console.error("Couldn\'t copy the text!");
  }
  frame.parentNode.removeChild(frame);
};