nextjs import but don't invoke function throws Module not found: Error: Can't resolve 'dns'

Question

My project(NextJS) was working fine and suddenly I am experiencing the issue ModuleNotFoundError. Particularly in the case of dynamic routing of nextJs.

Error I see is: Module not found: Error: Can't resolve 'dns'

In the pages directory pages/programs/[programtype]/[program].jsx when mongo is imported, it throws:

ModuleNotFoundError: Module not found: Error: Can't resolve 'dns' in 'node_modules/mongodb/lib'

Full error dump:

ModuleNotFoundError: Module not found: Error: Can't resolve 'dns' in '/project-path/node_modules/mongodb/lib'
    at /project-path/node_modules/webpack/lib/Compilation.js:925:10
    at /project-path/node_modules/webpack/lib/NormalModuleFactory.js:401:22
    at /project-path/node_modules/webpack/lib/NormalModuleFactory.js:130:21
    at /project-path/node_modules/webpack/lib/NormalModuleFactory.js:224:22
    at /project-path/node_modules/neo-async/async.js:2830:7
    at /project-path/node_modules/neo-async/async.js:6877:13
    at /project-path/node_modules/webpack/lib/NormalModuleFactory.js:214:25
    at /project-path/node_modules/enhanced-resolve/lib/Resolver.js:213:14
    at /project-path/node_modules/enhanced-resolve/lib/Resolver.js:285:5
    at eval (eval at create (/project-path/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:13:1)
    at /project-path/node_modules/enhanced-resolve/lib/UnsafeCachePlugin.js:44:7
    at /project-path/node_modules/enhanced-resolve/lib/Resolver.js:285:5
    at eval (eval at create (/project-path/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:13:1)
    at /project-path/node_modules/enhanced-resolve/lib/Resolver.js:285:5
    at eval (eval at create (/project-path/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:25:1)
    at /project-path/node_modules/enhanced-resolve/lib/DescriptionFilePlugin.js:67:43

Answer 1

The problem

This is a subtle problem with server-side code in Next.js.

The error is clear - you're trying to execute server side code (mongo query) in a client side code. But the cause is not obvious, because with Next.js you should be able to call Mongo from your components.

The cause

Next.js throws this error because you are importing your mongo code without using it.

It sounds weird but it is true.

How to avoid it

To avoid this error just remove any server-side code import in your components if you don't use it in getServerSideProps.

It sounds even more weird but it is true.

Good and bad examples

This works fine:

import { findUsers } from '../lib/queries'

function Home({ users }) {
  return (
    <h1>Users list</h1>
    //users.map and so on...
  )
}

export async function getServerSideProps() {
  const users = await findUsers()
  return {
    props: {
      users: users
    }
  }
}

export default Home

While this will throw the error:

import { findUsers } from '../lib/queries'

function Home({ users }) {
  return (
    <h1>Users list</h1>
    //users.map and so on...
  )
}

export async function getServerSideProps() {
  // call disabled to show the error
  // const users = await findUsers()
  return {
    props: {
      users: [] //returning an empty array to avoid other errors
    }
  }
}

export default Home