Rollup打包错误,onMounted中无法import？

RollupError: Invalid value "umd" for option "output.format" - UMD and IIFE output formats are not supported for code-splitting builds.

问题起因：vitepress内build的时候报错window is not defined,后查明是icon组件内iconfont.js有使用window。

改写为

<script lang="ts" setup>
import { computed, onMounted } from 'vue';
import './index.css';
// import './font/iconfont.js';

onMounted(() => {
    //@ts-ignore
    import('./font/iconfont.js').then(() => { });
})

rollup -c 打包会报错！！！

//rollup.config.js配置module.exports = {

input: './packages/index.ts',
output: {
    name: 'XXXXXXX',
    file: file('umd'),
    format: 'umd' // 编译模式
},
plugins: [
    nodeResolve(),
    typescript({ tsconfigOverride: overrides }),
    vuePlugin(),
    postcss({
        plugins: [autoprefixer()],
        extract: 'style.css'
    })
],
external: ['vue'] // 依赖模块

}