I meet a error that "default" is not exported....... #5777

I assume that you are using the @rollup/plugin-node-resolve to resolve third-party modules in node_modules. Since the package.json only includes the main field, and its value is dayjs.min.js, the dayjs import will be resolved to dayjs/dayjs.min.js. Unfortunately, dayjs/dayjs.min.js is not an ES module. One workaround is to add a custom plugin to resolve dayjs to dayjs/esm/index.js. Otherwise, you'll need to wait for the dayjs maintainer to add "exports": "esm/index.js" to the package.json.

This issue is caused by the module format of the dayjs package. As @TrickyPi mentioned, dayjs only has main: "dayjs.min.js" in its package.json, which is CommonJS/UMD format, but element-plus ESM expects an ESM default export.

Solution 1: Use @rollup/plugin-alias to redirect to ESM version

dayjs actually ships with an ESM version at dayjs/esm/index.js, it just isn't declared in package.json.

import alias from '@rollup/plugin-alias';

export default {
  plugins: [
    alias({
      entries: [
        { find: 'dayjs', replacement: 'dayjs/esm/index.js' }
      ]
    }),
    // ... other plugins
  ]
};

Solution 2: Custom resolve plugin

const resolveDayjsEsm = {
  name: 'resolve-dayjs-esm',
  resolveId(source) {
    if (source === 'dayjs') {
      return { id: 'dayjs/esm/index.js', external: false };
    }
    return null;
  }
};

export default {
  plugins: [
    resolveDayjsEsm,
    // ... other plugins (put this BEFORE node-resolve)
  ]
};

Note: If you also use dayjs plugins (e.g., dayjs/plugin/customParseFormat), you'll need to map them to their ESM paths as well: dayjs/esm/plugin/customParseFormat.

ese error pasa porque dayjs no exporta un default en el formato que espera element-plus. es un problema conocido de compatibilidad entre cjs y esm. en rollup puedes solucionarlo con el plugin commonjs:

import commonjs from "@rollup/plugin-commonjs"

plugins: [
  commonjs({
    include: /node_modules/,
    requireReturnsDefault: "auto"
  })
]

la opcion requireReturnsDefault en auto hace que cuando un modulo cjs no tiene default export lo sintetice automaticamente. si eso no funciona prueba con esmExternals: true tambien. es un lio este tema de cjs vs esm con dayjs porque segun la version que uses cambia como exporta