This helper script bridges compatibility between the Greasemonkey 4 APIs and existing/legacy APIs.

gm4-polyfill.module.js

/*
This helper script bridges compatibility between the Greasemonkey 4 APIs and
existing/legacy APIs.  Say for example your user script includes

    // @grant GM_getValue

And you'd like to be compatible with both Greasemonkey 4 and Greasemonkey 4
(and for that matter all versions of Violentmonkey, Tampermonkey, and any other
user script engine).  Add:

    // @grant GM.getValue
    // @require https://arantius.com/misc/greasemonkey/imports/greasemonkey4-polyfill.js

And switch to the new (GM-dot) APIs, which return promises.  If your script
is running in an engine that does not provide the new asynchronous APIs, this
helper will add them, based on the old APIs.

If you use `await` at the top level, you'll need to wrap your script in an `async`
function to be compatible with any user script engine besides Greasemonkey 4.

    (async () => {
    let x = await GM.getValue('x');
    })();
*/

module.exports = (function() {
	if (typeof GM == 'undefined') {
		GM = {'log': console.log};
	}


	if (typeof GM_addStyle == 'undefined') {
		function GM_addStyle(aCss) {
			'use strict';
			let head = document.getElementsByTagName('head')[0];
			if (head) {
				let style = document.createElement('style');
				style.setAttribute('type', 'text/css');
				style.textContent = aCss;
				head.appendChild(style);
				return style;
			}
			return null;
		}
	}
	GM.addStyle = GM_addStyle;


	if (typeof GM_registerMenuCommand == 'undefined') {
		function GM_registerMenuCommand(caption, commandFunc, accessKey) {
			if (!document.body) {
				console.error('GM_registerMenuCommand got no body.');
				return;
			}
			let menu = document.getElementById('gm-registered-menu');
			if (!menu) {
				menu = document.createElement('menu')
				menu.setAttribute('id', 'gm-registered-menu');
				menu.setAttribute('type', 'context');
				document.body.appendChild(menu);
				document.body.setAttribute('contextmenu', 'gm-registered-menu');
			}
			let menuItem = document.createElement('menuitem');
			menuItem.textContent = caption;
			menuItem.addEventListener('click', commandFunc, true);
			menu.appendChild(menuItem);
		}
	}
	GM.registerMenuCommand = GM_registerMenuCommand;


	GM.info = GM_info;


	Object.entries({
		'GM_deleteValue': 'deleteValue',
		'GM_getResourceURL': 'getResourceUrl',
		'GM_getValue': 'getValue',
		'GM_listValues': 'listValues',
		'GM_notification': 'notification',
		'GM_openInTab': 'openInTab',
		'GM_setClipboard': 'setClipboard',
		'GM_setValue': 'setValue',
		'GM_xmlhttpRequest': 'xmlHttpRequest',
	}).forEach(([oldKey, newKey]) => {
		let old = window[oldKey] || this[oldKey];
		if (old) GM[newKey] = function() {
			return new Promise((resolve, reject) => {
				try {
					resolve(old.apply(this, arguments));
				} catch (e) {
					reject(e);
				}
			});
		}
	});
	return GM;
})();

greasemonkey4-polyfill.js

/*
This helper script bridges compatibility between the Greasemonkey 4 APIs and
existing/legacy APIs.  Say for example your user script includes

    // @grant GM_getValue

And you'd like to be compatible with both Greasemonkey 4 and Greasemonkey 4
(and for that matter all versions of Violentmonkey, Tampermonkey, and any other
user script engine).  Add:

    // @grant GM.getValue
    // @require https://arantius.com/misc/greasemonkey/imports/greasemonkey4-polyfill.js

And switch to the new (GM-dot) APIs, which return promises.  If your script
is running in an engine that does not provide the new asynchronous APIs, this
helper will add them, based on the old APIs.

If you use `await` at the top level, you'll need to wrap your script in an `async`
function to be compatible with any user script engine besides Greasemonkey 4.

    (async () => {
    let x = await GM.getValue('x');
    })();
*/

if (typeof GM == 'undefined') {
  GM = {'log': console.log};
}


if (typeof GM_addStyle == 'undefined') {
  function GM_addStyle(aCss) {
    'use strict';
    let head = document.getElementsByTagName('head')[0];
    if (head) {
      let style = document.createElement('style');
      style.setAttribute('type', 'text/css');
      style.textContent = aCss;
      head.appendChild(style);
      return style;
    }
    return null;
  }
}
GM.addStyle = GM_addStyle;


if (typeof GM_registerMenuCommand == 'undefined') {
  function GM_registerMenuCommand(caption, commandFunc, accessKey) {
    if (!document.body) {
      console.error('GM_registerMenuCommand got no body.');
      return;
    }
    let menu = document.getElementById('gm-registered-menu');
    if (!menu) {
      menu = document.createElement('menu')
      menu.setAttribute('id', 'gm-registered-menu');
      menu.setAttribute('type', 'context');
      document.body.appendChild(menu);
      document.body.setAttribute('contextmenu', 'gm-registered-menu');
    }
    let menuItem = document.createElement('menuitem');
    menuItem.textContent = caption;
    menuItem.addEventListener('click', commandFunc, true);
    menu.appendChild(menuItem);
  }
}
GM.registerMenuCommand = GM_registerMenuCommand;


GM.info = GM_info;


Object.entries({
  'GM_deleteValue': 'deleteValue',
  'GM_getResourceURL': 'getResourceUrl',
  'GM_getValue': 'getValue',
  'GM_listValues': 'listValues',
  'GM_notification': 'notification',
  'GM_openInTab': 'openInTab',
  'GM_setClipboard': 'setClipboard',
  'GM_setValue': 'setValue',
  'GM_xmlhttpRequest': 'xmlHttpRequest',
}).forEach(([oldKey, newKey]) => {
  let old = this[oldKey];
  if (old) GM[newKey] = function() {
    return new Promise((resolve, reject) => {
      try {
        resolve(old.apply(this, arguments));
      } catch (e) {
        reject(e);
      }
    });
  }
});