Support ESM in next.config.js

Question

I'm making some optimation on nextjs project and need to has type: 'module' on thepackage.json file. But then got the error

Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: my_path/next.config.js require() of ES modules is not supported.

Seem next.config.js is not support ESM yet. The issue already discussed here: https://github.com/vercel/next.js/issues/9607 but I can find a solution yet. Any help guys?

I'm using: node v12.17.0 next 11.1.0

And here is my next.config.js

import withLess from '@zeit/next-less'

const nextConfig = {
  target: 'serverless',
  productionBrowserSourceMaps: true,
  webpack5: true,
  onDemandEntries: {
    maxInactiveAge: 1000 * 60 * 60,
    pagesBufferLength: 5
  },
  lessLoaderOptions: {
    javascriptEnabled: true
  },
  trailingSlash: false,
}

export default withLess(nextConfig)

My package.json file

{
  "type": "module"
  ...
}

UPDATE: What I optimated is changing the way to call Component from 'ant' package.

Form

import { Row, Col } from 'antd'

To

import Row from 'antd/es/row'
import Col from 'antd/es/col'

then cause this error

my_path/node_modules/antd/es/row/index.js:1

import { Row } from '../grid'; ^^^^^^

SyntaxError: Cannot use import statement outside a module

I fixed this by add type: "module" in package.json and have problem with next.config.js file

Maybe this can help you? stackoverflow.com/questions/65974337/… — Camilo
– Camilo, Commented Aug 26, 2021 at 18:44
"I'm making some optimizations on Next.js project and need to has type: 'module'" -- can I know what optimizations you are referring to? Cause IMHO only files related with your custom server are not processed by babel/webpack, and hence don't support ESM syntax by default, but can be made so by simply changing their extension to .mjs. What is the need to modify package.json to indicate type: module at the first place? — brc-dd
– brc-dd, Commented Aug 31, 2021 at 14:38
@bird So I guess you did that because for some reasons antd's tree shaking wasn't working for you. Right? — brc-dd
– brc-dd, Commented Sep 1, 2021 at 10:33
@brc-dd yes, right. I am trying another way. That's config the webpack — tam.dangc
– tam.dangc, Commented Sep 1, 2021 at 10:54

juliomalves · Accepted Answer · 2021-10-30 18:58:09Z

30

From Next.js 12, ES modules is now supported in the config file by renaming it to next.config.mjs.

// next.config.mjs
import withLess from '@zeit/next-less'

export default withLess({
    productionBrowserSourceMaps: true,
    onDemandEntries: {
        maxInactiveAge: 1000 * 60 * 60,
        pagesBufferLength: 5
    },
    lessLoaderOptions: {
        javascriptEnabled: true
    },
    trailingSlash: false
})

answered Oct 30, 2021 at 18:58

juliomalves

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2 Comments

Raphael PICCOLO Over a year ago

having next.config.mjs and type: 'module' in package.json results in error: require() of ES Module /usr/app/.next/server/pages/_document.js from /usr/app/node_modules/next/dist/server/require.js not supported.

machineghost Over a year ago

You only need to pick one or the other: you can either rename every file to .mjs or you can add "type": "module" in package.json.

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