Unable to identify what's happening in my next.js app. As fs is a default file system module of nodejs. It is giving the error of module not found.
If you use fs, be sure it's only within getInitialProps or getServerSideProps. (anything includes server-side rendering).
You may also need to create a next.config.js file with the following content to get the client bundle to build:
For webpack4
module.exports = {
webpack: (config, { isServer }) => {
// Fixes npm packages that depend on `fs` module
if (!isServer) {
config.node = {
fs: 'empty'
}
}
return config
}
}
For webpack5
module.exports = {
webpack5: true,
webpack: (config) => {
config.resolve.fallback = { fs: false };
return config;
},
};
Note: for other modules such as path, you can add multiple arguments such as
{
fs: false,
path: false
}
I spent hours on this and the solution is also here on Stackoverflow but on different issue -> https://stackoverflow.com/a/67478653/17562602
Hereby I asked for MOD permission to reshare this, since this issue is the first one to show up on Google and probably more and more people stumble would upon the same problem as I am, so I'll try to saved them some sweats
Soo, You need to add this in your next.config.js
module.exports = {
future: {
webpack5: true, // by default, if you customize webpack config, they switch back to version 4.
// Looks like backward compatibility approach.
},
webpack(config) {
config.resolve.fallback = {
...config.resolve.fallback, // if you miss it, all the other options in fallback, specified
// by next.js will be dropped. Doesn't make much sense, but how it is
fs: false, // the solution
};
return config;
},
};
It works for like a charm for me
Minimal reproducible example
A clean minimal example will be beneficial to Webpack beginners since auto splitting based on usage is so mind-blowingly magic.
Working hello world baseline:
pages/index.js
// Client + server code.
export default function IndexPage(props) {
return <div>{props.msg}</div>
}
// Server-only code.
export function getStaticProps() {
return { props: { msg: 'hello world' } }
}
package.json
{
"name": "test",
"version": "1.0.0",
"scripts": {
"dev": "next",
"build": "next build",
"start": "next start"
},
"dependencies": {
"next": "12.0.7",
"react": "17.0.2",
"react-dom": "17.0.2"
}
}
Run with:
npm install
npm run dev
Now let's add a dummy require('fs') to blow things up:
// Client + server code.
export default function IndexPage(props) {
return <div>{props.msg}</div>
}
// Server-only code.
const fs = require('fs')
export function getStaticProps() {
return { props: { msg: 'hello world' } }
}
fails with:
Module not found: Can't resolve 'fs'
which is not too surprising, since there was no way for Next.js to know that that fs was server only, and we wouldn't want it to just ignore random require errors, right? Next.js only knows that for getStaticProps because that's a hardcoded Next.js function name.
OK, so let's inform Next.js by using fs inside getStaticProps, the following works again:
// Client + server code.
export default function IndexPage(props) {
return <div>{props.msg}</div>
}
// Server-only code.
const fs = require('fs')
export function getStaticProps() {
fs
return { props: { msg: 'hello world' } }
}
Mind equals blown. So we understand that any mention of fs inside of the body of getStaticProps, even an useless one like the above, makes Next.js/Webpack understand that it is going to be server-only.
Things would work the same for getServerSideProps and getStaticPaths.
Higher order components (HOCs) have to be in their own files
Now, the way that we factor out IndexPage and getStaticProps across different but similar pages is to use HOCs, which are just functions that return other functions.
HOCs will normally be put outside of pages/ and then required from multiple locations, but when you are about to factor things out to generalize, you might be tempted to put them directly in the pages/ file temporarily, something like:
// Client + server code.
import Link from 'next/link'
export function makeIndexPage(isIndex) {
return (props) => {
return <>
<Link href={isIndex ? '/index' : '/notindex'}>
<a>{isIndex ? 'index' : 'notindex'}</a>
</Link>
<div>{props.fs}</div>
<div>{props.isBlue}</div>
</>
}
}
export default makeIndexPage(true)
// Server-only code.
const fs = require('fs')
export function makeGetStaticProps(isBlue) {
return () => {
return { props: {
fs: Object.keys(fs).join(' '),
isBlue,
} }
}
}
export const getStaticProps = makeGetStaticProps(true)
but if you do this you will be saddened to see:
Module not found: Can't resolve 'fs'
So we understand another thing: the fs usage has to be directly inside the getStaticProps function body, Webpack can't catch it in subfunctions.
The only way to solve this is to have a separate file for the backend-only stuff as in:
pages/index.js
// Client + server code.
import { makeIndexPage } from "../front"
export default makeIndexPage(true)
// Server-only code.
import { makeGetStaticProps } from "../back"
export const getStaticProps = makeGetStaticProps(true)
pages/notindex.js
// Client + server code.
import { makeIndexPage } from "../front"
export default makeIndexPage(false)
// Server-only code.
import { makeGetStaticProps } from "../back"
export const getStaticProps = makeGetStaticProps(false)
front.js
// Client + server code.
import Link from 'next/link'
export function makeIndexPage(isIndex) {
return (props) => {
console.error('page');
return <>
<Link href={isIndex ? '/notindex' : '/'}>
<a>{isIndex ? 'notindex' : 'index'}</a>
</Link>
<div>{props.fs}</div>
<div>{props.isBlue}</div>
</>
}
}
back.js
// Server-only code.
const fs = require('fs')
export function makeGetStaticProps(isBlue) {
return () => {
return { props: {
fs: Object.keys(fs).join(' '),
isBlue,
} }
}
}
Webpack must see that name makeGetStaticProps getting assigned to getStaticProps, so it decides that the entire back file is server-only.
Note that it does not work if you try to merge back.js and front.js into a single file, probably because when you do export default makeIndexPage(true) webpack necessarily tries to pull the entire front.js file into the frontend, which includes the fs, so it fails.
This leads to a natural (and basically almost mandatory) split of library files between:
front.js and front/*: front-end + backend files. These are safe for the frontend. And the backend can do whatever the frontend can do (we are doing SSR right?) so those are also usable from the backend.
Perhaps this is the idea behind the conventional "components" folder in many official examples. But that is a bad name, because that folder should not only contain components, but also any library non-component helpers/constants that will be used from the frontend.
back.js and back/* (or alternatively anything outside of front/*): backend only files. These can only be used by the backend, importing them on frontend will lead to the error
fs,path or other node native modules can be used only inside server-side code, like "getServerSide" functions. If you try to use it in client you get error even you just console.log it.. That console.log should run inside server-side functions as well.
When you import "fs" and use it in server-side, next.js is clever enough to see that you use it in server-side so it wont add that import into the client bundle
One of the packages that I used was giving me this error, I fixed this with
module.exports = {
webpack: (config, { isServer }) => {
if (!isServer) {
config.resolve.fallback.fs = false
}
return config
},
}
but this was throwing warning on terminal:
"Critical dependency: require function is used in a way in which
dependencies cannot be statically extracted"
Then I tried to load the node module on the browser. I copied the "min.js" of the node module from the node_modules and placed in "public/js/myPackage.js" and load it with Script
export default function BaseLayout({children}) {
return (
<>
<Script
// this in public folder
src="/js/myPackage.js"
// this means this script will be loaded first
strategy="beforeInteractive"
/>
</>
)
}
This package was attached to window object and in node_modules source code's index.js:
if (typeof window !== "undefined") {
window.TruffleContract = contract;
}
So I could access to this script as window.TruffleContract. BUt this was not an efficient way.
While this error requires a bit more reasoning than most errors you'll encounter, it happens for a straightforward reason.
Why this happens
Next.js, unlike many frameworks allows you to import server-only (Node.js APIs that don't work in a browser) code into your page files. When Next.js builds your project, it removes server only code from your client-side bundle by checking which code exists inside one any of the following built-in methods (code splitting):
getServerSideProps
getStaticProps
getStaticPaths
Side note: there is a demo app that visualizes how this works.
The Module not found: can't resolve 'xyz' error happens when you try to use server only code outside of these methods.
Error example 1 - basic
To reproduce this error, let's start with a working simple Next.js page file.
WORKING file
/** THIS FILE WORKS FINE! */
import type { GetServerSideProps } from "next";
import fs from "fs"; // our server-only import
type Props = {
doesFileExist: boolean;
};
export const getServerSideProps: GetServerSideProps = async () => {
const fileExists = fs.existsSync("/some-file");
return {
props: {
doesFileExist: fileExists,
},
};
};
const ExamplePage = ({ doesFileExist }: Props) => {
return <div>File exists?: {doesFileExist ? "Yes" : "No"}</div>;
};
export default ExamplePage;
Now, let's reproduce the error by moving our fs.existsSync method outside of getServerSideProps. The difference is subtle, but the code below will throw our dreaded Module not found error.
ERROR file
import type { GetServerSideProps } from "next";
import fs from "fs";
type Props = {
doesFileExist: boolean;
};
/** ERROR!! - Module not found: can't resolve 'fs' */
const fileExists = fs.existsSync("/some-file");
export const getServerSideProps: GetServerSideProps = async () => {
return {
props: {
doesFileExist: fileExists,
},
};
};
const ExamplePage = ({ doesFileExist }: Props) => {
return <div>File exists?: {doesFileExist ? "Yes" : "No"}</div>;
};
export default ExamplePage;
Error example 2 - realistic
The most common (and confusing) occurrence of this error happens when you are using modules that contain multiple types of code (client-side + server-side).
Let's say I have the following module called file-utils.ts:
import fs from 'fs'
// This code only works server-side
export function getFileExistence(filepath: string) {
return fs.existsSync(filepath)
}
// This code works fine on both the server AND the client
export function formatResult(fileExistsResult: boolean) {
return fileExistsResult ? 'Yes, file exists' : 'No, file does not exist'
}
In this module, we have one server-only method and one "shared" method that in theory should work client-side (but as we'll see, theory isn't perfect).
Now, let's try incorporating this into our Next.js page file.
/** ERROR!! */
import type { GetServerSideProps } from "next";
import { getFileExistence, formatResult } from './file-utils.ts'
type Props = {
doesFileExist: boolean;
};
export const getServerSideProps: GetServerSideProps = async () => {
return {
props: {
doesFileExist: getFileExistence('/some-file')
},
};
};
const ExamplePage = ({ doesFileExist }: Props) => {
// ERROR!!!
return <div>File exists?: {formatResult(doesFileExist)}</div>;
};
export default ExamplePage;
As you can see, we get an error here because when we attempt to use formatResult client-side, our module still has to import the server-side code.
To fix this, we need to split our modules up into two categories:
Server only
Shared code (client or server)
// file-utils.ts
import fs from 'fs'
// This code (and entire file) only works server-side
export function getFileExistence(filepath: string) {
return fs.existsSync(filepath)
}
// file-format-utils.ts
// This code works fine on both the server AND the client
export function formatResult(fileExistsResult: boolean) {
return fileExistsResult ? 'Yes, file exists' : 'No, file does not exist'
}
Now, we can create a WORKING page file:
/** WORKING! */
import type { GetServerSideProps } from "next";
import { getFileExistence } from './file-utils.ts' // server only
import { formatResult } from './file-format-utils.ts' // shared
type Props = {
doesFileExist: boolean;
};
export const getServerSideProps: GetServerSideProps = async () => {
return {
props: {
doesFileExist: getFileExistence('/some-file')
},
};
};
const ExamplePage = ({ doesFileExist }: Props) => {
return <div>File exists?: {formatResult(doesFileExist)}</div>;
};
export default ExamplePage;
Solutions
There are 2 ways to solve this:
The "correct" way
The "just get it working" way
The "Correct" way
The best way to solve this error is to make sure that you understand why it is happening (above) and make sure you are only using server-side code inside getStaticPaths, getStaticProps, or getServerSideProps and NOWHERE else.
And remember, if you import a module that contains both server-side and client-side code, you cannot use any of the imports from that module client-side (revisit example #2 above).
The "Just get it working" way
As others have suggested, you can alter your next.config.js to ignore certain modules at build-time. This means that when Next.js attempts to split your page file between server only and shared code, it will not try to polyfill Node.js APIs that fail to build client-side.
In this case, you just need:
/** next.config.js - with Webpack v5.x */
module.exports = {
... other settings ...
webpack: (config, { isServer }) => {
// If client-side, don't polyfill `fs`
if (!isServer) {
config.resolve.fallback = {
fs: false,
};
}
return config;
},
};
Drawbacks of this approach
As shown in the resolve.fallback section of the Webpack documentation, the primary reason for this config option is because as-of Webpack v5.x, core Node.js modules are no longer polyfilled by default. Therefore, the main purpose for this option is to provide a way for you to define which polyfill you want to use.
When you pass false as an option, this means, "do not include a polyfill".
While this works, it can be fragile and require ongoing maintenance to include any new modules that you introduce to your project. Unless you are converting an existing project / supporting legacy code, it is best to go for option #1 above as it promotes better module organization according to how Next.js actually splits the code under the hood.
If trying to use fs-extra in Next.js, this worked for me
module.exports = {
webpack: (config) => {
config.resolve.fallback = { fs: false, path: false, stream: false, constants: false };
return config;
}
}
I got this error in my NextJS app because I was missing export in
export function getStaticProps()
/** #type {import('next').NextConfig} */
module.exports = {
reactStrictMode: false,
webpack5: true,
webpack: (config) => {
config.resolve.fallback = {
fs: false,
net: false,
dns: false,
child_process: false,
tls: false,
};
return config;
},
};
This code fixed my problem and I want to share.Add this code to your next.config file.i'm using
webpack5
For me clearing the cache
npm cache clean -f
and then updating the node version to the latest stable release(14.17.0) worked
It might be that the module you are trying to implement is not supposed to run in a browser. I.e. it's server-side only.
For me, the problem was the old version of the node.js installed. It requires node.js version 14 and higher. The solution was to go to the node.js web page, download the latest version and just install it. And then re-run the project. All worked!
I had the same issue when I was trying to use babel.
For me this worked:
#add a .babelrc file to the root of the project and define presets and plugins
(in my case, I had some issues with the macros of babel, so I defined them)
{
"presets": ["next/babel"],
"plugins": ["macros"]
}
after that shut down your server and run it again
I had this exact issue. My problem was that I was importing types that I had declared in a types.d.ts file.
I was importing it like this, thanks to the autofill provided by VSCode.
import {CUSTOM_TYPE} from './types'
It should have been like this:
import {CUSTOM_TYPE} from './types.d'
In my case, I think the .d was unnecessary so I ended up removing it entirely and renamed my file to types.ts.
Weird enough, it was being imported directly into index.tsx without issues, but any helper files/functions inside the src directory would give me errors.
I ran into this in a NextJS application because I had defined a new helper function directly below getServerSideProps(), but had not yet called that function inside getServerSideProps().
I'm not sure why this created a problem, but it did. I could only get it to work by either calling that function, removing it, or commenting it out.
Don't use fs in the pages directory, since next.js suppose that files in pages directory are running in browser environment.
You could put the util file which uses fs to other directory such as /core
Then require the util in getStaticProps which runs in node.js environment.
// /pages/myPage/index.tsx
import View from './view';
export default View;
export async function getStaticProps() {
const util = require('core/some-util-uses-fs').default; // getStaticProps runs in nodes
const data = await util.getDataFromDisk();
return {
props: {
data,
},
};
}
In my case, this error appeared while refactoring the auth flow of a Next.js page. The cause was some an unused imports that I had not yet removed.
Previously I made the page a protected route like so:
export async function getServerSideProps ({ query, req, res }) {
const session = await unstable_getServerSession(req, res, authOptions)
if (!session) {
return {
redirect: {
destination: '/signin',
permanent: false,
},
}
}
//... rest of server-side logic
}
Whilst refactoring, I read up on NextAuth useSession. Based on what I read there, I was able to change the implementation such that I simply needed to add
MyComponent.auth = true to make a page protected. I then deleted the aforementioned code block inside of getServerSideProps. However, I had not yet deleted the two imports used by said code block:
import { unstable_getServerSession } from 'next-auth/next'
import { authOptions } from 'pages/api/auth/[...nextauth]'
I believe the second of those two imports was causing the problem. So the summary is that in addition to all of the great answers above, it could also be an unused import.
Sometimes this error can be because you have imported something but not mastered it anywhere. This worked for me. I reviewed my code and removed the unused dependencies.
Super stumped by this. I have some server code that for some reason throws a UTF-8 error in my tests but works fine when running the server normally:
code:
export default ({ projectId = PROJECT_ID, esHost = ES_HOST } = {}) => {
let app = express();
app.use(cors());
app.use(bodyParser.json({ limit: '50mb' }));
let http = Server(app);
let io = socketIO(http);
let server = {
app,
io,
http,
status: 'off',
listen(
port = PORT,
cb = () => {
rainbow(`⚡️ Listening on port ${port} ⚡️`);
},
) {
this.http.listen(port, () => {
main({ io, app, projectId, esHost, port });
this.status = 'on';
cb();
});
},
close(cb = () => {}) {
if (this.http) {
this.http.close(() => {
this.status = 'off';
cb();
});
} else {
throw '❗️ cannot close server that has not been started ❗️';
}
},
};
return server;
};
usage (exactly the same, but in jest test body-parser isn't working properly):
import createServer from '../server'
let server = createServer()
server.listen(5050);
I'm using postman, post response outside of test:
{
"projects": [
{
"id": "test",
"active": true,
"timestamp": "2018-02-25T21:33:08.006Z"
},
{
"id": "TEST-PROJECT",
"active": true,
"timestamp": "2018-03-05T21:34:34.604Z"
},
{
"id": "asd",
"active": true,
"timestamp": "2018-03-06T23:29:55.348Z"
}
],
"total": 3
}
unexpected post response inside jest test server:
Error
UnsupportedMediaTypeError: unsupported charset "UTF-8" at /Users/awilmer/Projects/arranger/node_modules/body-parser/lib/read.js:83:18 at invokeCallback (/Users/awilmer/Projects/arranger/node_modules/raw-body/index.js:224:16) at _combinedTickCallback (internal/process/next_tick.js:131:7) at process._tickCallback (internal/process/next_tick.js:180:9)
So I was able to reproduce the issue and find the source of the issue and the workaround to make it work. The issue is caused by jest framework.
Before you jump on reading the rest of the thread, I would suggest you read another Jest thread I answer long back. This would help get some context internals about the require method in jest
Specify code to run before any Jest setup happens
Cause
The issue happens only in test and not in production. This is because of jest require method.
When you run your tests, it starts a express server, which calls the node_modules/raw-body/index.js as shown in below image
As you can see the encodings is null. This is because the iconv-lite module does a lazy loading of encodings. The encodings are only loaded when getCodec method gets executed.
Now when your test has fired the API, the server needs to read the body so the getCodec gets called
This then goes through the jest-runtime/build/index.js custom require method (which is overloaded if you read the previous link).
The execModule has a check for this._environment.global, which is blank in this case and hence a null value is returned and the module never gets executed
Now when you look at the exports of the encodings module, it just is a blank object
So the issue is purely a jest. A feature jest lacks or a bug mostly?
Related Issues
Related issues have already been discussed on below threads
https://github.com/facebook/jest/issues/2605
https://github.com/RubenVerborgh/N3.js/issues/120
https://github.com/sidorares/node-mysql2/issues/489#issuecomment-313374683
https://github.com/ashtuchkin/iconv-lite/issues/118
https://github.com/Jason-Rev/vscode-spell-checker/issues/159
Fix
The fix to the problem is that we load the module during our test itself and force a early loading instead of lazy loading. This can be done by adding a line to your index.test.js at the top
import encodings from '../../node_modules/iconv-lite/encodings';
import createServer from '#arranger/server';
After the change all the test pass, though you have a error in the url of the test so you get Cannot POST /
I'm adding a slightly different solution inspired from #Tarun Lalwani
Add the following lines at the top of your test file.
const encodings = require('./node_modules/iconv-lite/encodings');
const iconvLite = require('./node_modules/iconv-lite/lib');
iconvLite.getCodec('UTF-8');
I spent many hours trying to figure out why Jest would report a 415 error code when testing the Node.js server. Node.js is configured to use app.use(bodyParser.json(...)); on our system, too. That didn't solve the issue.
Solution
When using res.status(...), you MUST either chain on .json() or use res.json(), too. That means if you respond with a 500 error or otherwise and you don't return any JSON data, you still need to use res.json(). No idea why, as that defeats the whole purpose of app.use(bodyParser.json(...)); in the first place.
Example
const express = require('express');
const router = express.Router();
router.post("/register", (req, res) => {
// ...
res.status(500).json();
// ...
});
How do I configure jest tests to fail on warnings?
console.warn('stuff');
// fail test
You can use this simple override :
let error = console.error
console.error = function (message) {
error.apply(console, arguments) // keep default behaviour
throw (message instanceof Error ? message : new Error(message))
}
You can make it available across all tests using Jest setupFiles.
In package.json :
"jest": {
"setupFiles": [
"./tests/jest.overrides.js"
]
}
Then put the snippet into jest.overrides.js
For those using create-react-app, not wanting to run npm run eject, you can add the following code to ./src/setupTests.js:
global.console.warn = (message) => {
throw message
}
global.console.error = (message) => {
throw message
}
Now, jest will fail when messages are passed to console.warn or console.error.
create-react-app Docs - Initializing Test Environment
I implemented this recently using jest.spyOn introduced in v19.0.0 to mock the warn method of console (which is accesses via the global context / object).
Can then expect that the mocked warn was not called, as shown below.
describe('A function that does something', () => {
it('Should not trigger a warning', () => {
var warn = jest.spyOn(global.console, 'warn');
// Do something that may trigger warning via `console.warn`
doSomething();
// ... i.e.
console.warn('stuff');
// Check that warn was not called (fail on warning)
expect(warn).not.toHaveBeenCalled();
// Cleanup
warn.mockReset();
warn.mockRestore();
});
});
There is a useful npm package that helps you to achieve that: jest-fail-on-console
It's easily configurable.
Install:
npm i -D jest-fail-on-console
Configure:
In a file used in the setupFilesAfterEnv option of Jest, add this code:
import failOnConsole from 'jest-fail-on-console'
failOnConsole()
// or with options:
failOnConsole({ shouldFailOnWarn: false })
I decided to post a full example based on user1823021 answer
describe('#perform', () => {
var api
// the global.fetch is set to jest.fn() object globally
global.fetch = jest.fn()
var warn = jest.spyOn(global.console, 'warn');
beforeEach(function() {
// before every test, all mocks need to be resetted
api = new Api()
global.fetch.mockReset()
warn.mockReset()
});
it('triggers an console.warn if fetch fails', function() {
// In this test fetch mock throws an error
global.fetch.mockImplementationOnce(() => {
throw 'error triggered'
})
// I run the test
api.perform()
// I verify that the warn spy has been triggered
expect(warn).toHaveBeenCalledTimes(1);
expect(warn).toBeCalledWith("api call failed with error: ", "error triggered")
});
it('calls fetch function', function() {
// I create 2 more mock objects to verify the fetch parameters
const url = jest.fn()
const config = jest.fn()
api.url = url
api.config = config
// I run the test
api.perform()
// I verify that fetch has been called with url and config mocks
expect(global.fetch).toHaveBeenCalledTimes(1)
expect(global.fetch).toBeCalledWith(url, config)
expect(warn).toHaveBeenCalledTimes(0)
});
})
the #perform method I am testing
class Api {
constructor(auth) {
this._credentials = auth
}
perform = async () => {
try {
return await fetch(this.url, this.config)
} catch(error) {
console.warn('api call failed with error: ', error)
}
}
}
You can set the environment variable CI=true before running jest which will cause it to fail tests on warnings in addition to errors.
Example which runs all test files in the test folder:
CI=true jest ./test
Automated CI/CD pipelines such as Github Actions set CI to true by default, which can be one reason why a unit test will pass on your local machine when warnings are thrown, but fail in the pipeline.
(Here is the Github Actions documentation on default environment variables: https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables)