isLoggedIn().then(main); // run main() when user logged in

let jsonrpc = new JSONRPC(); // JSONRPC client, Promise based

function isLoggedIn() {
	return jsonrpc.call('User.isLoggedIn').catch(showLoginForm);
}

function logIn(args) {
	return jsonrpc.call('User.logIn', args).catch(showLoginForm);
}

function showLoginForm(error) {
	return new Promise(resolve => {
		let mbox = new MessageBox( //API module, not promising, event based
      			`Username: <input name="username"/><br/>
       		 	 Password: <input name="password" type="password"/>`,
      			`ERROR: ${error.message}`
    		);
		mbox.actionEvent.attach(MB.IDOK, (msgbox, formData) => logIn(formData).then(resolve));
		mbox.actionEvent.attach(MB.IDCLOSE, () => showLoginForm(new Error('Close button not allowed')).then(resolve));
    		mbox.show();
	})
}

function main(data) {  //data - return value of isLoggedIn/logIn function
  // do important stuff
}