// ./messaging.ts
import { defineExtensionMessaging } from '@webext-core/messaging';

interface ProtocolMap {
  getStringLength(s: string): number;
}

export const { sendMessage, onMessage } = defineExtensionMessaging<ProtocolMap>();

// ./background.ts
import { onMessage } from './messaging';

onMessage('getStringLength', message => {
  return message.data.length;
});

// ./content-script.js or anywhere else
import { sendMessage } from './messaging';

const length = await sendMessage('getStringLength', 'hello world');

console.log(length); // 11