How to use vue-i18n in ts files

[QC2168]

I using i18n.global.t function to translate, it normal working

However, I received a typescript error, how can I resolve it?

[QC2168]

// this is my code
import type { App } from 'vue';
import type { I18nOptions } from 'vue-i18n';
import cn from './lang/zh-CN/index.json'
import en from './lang/en/index.json'
import { createI18n } from 'vue-i18n';

export let i18n: ReturnType<typeof createI18n>;

export const t = (key: string) => i18n.global.t(key);

const messages={
  cn,en
}
async function createI18nOptions(): Promise<I18nOptions> {
  return {
    legacy: false,
    globalInjection: true, // 全局注册$t方法
    locale: 'cn',
    messages
  };
}

// setup i18n instance with glob
export async function setupI18n(app: App) {
  const options = await createI18nOptions();
  i18n = createI18n(options);
  app.use(i18n);
}