Client bundles contain non es5 code

[tfkhim]

Run next info (available from version 12.0.8 and up)

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT Wed, 05 Jan 2022 16:20:59 +0000
Binaries:
  Node: 16.7.0
  npm: 7.20.3
  Yarn: N/A
  pnpm: N/A
Relevant packages:
  next: 12.0.8
  react: 17.0.2
  react-dom: 17.0.2

What version of Next.js are you using?

12.0.8

What version of Node.js are you using?

16.7.0

What browser are you using?

Chromium, Firefox

What operating system are you using?

Linux

How are you deploying your application?

next start

Describe the Bug

The update to 12.0.8 introduced a regression w.r.t. es5 compatibility. If running the command series given below the following error occurs:

ES-Check Error:
          ----
          · erroring file: ./.next/static/chunks/main-1e7421553b9673ee.js
          · error: SyntaxError: The keyword 'const' is reserved (1:97874)
          · see the printed err.stack below for context
          ----

          SyntaxError: The keyword 'const' is reserved (1:97874)

I think this is related to the changes in #33159 which added a new method getProperError to the ìs-error.js file. This method is used on the client side in next/client/index.tsx. The previously used isError function was very simple and es5 compatible. The new method uses the isPlainObject method which contains a const declaration.

I think the root cause is that dist/lib isn't transpiled during building. Looking at the Babel loader regex only dist/shared/lib seems to be transpiled. So moving those files there might be a solution for this problem.

Expected Behavior

The Webpack bundle files contain only es5 compatible JavaScript code.

To Reproduce

• npx create-next-app@latest --typescript
• npx next build
• npx es-check es5 './.next/static/**/*.js'

[balazsorban44]

Hi, I just double-checked and const is partially supported in IE11 https://caniuse.com/const, which covers our use case of it.

You can verify this by visiting https://33314.vercel.app in IE11 and it will work fine.

That's the lowest possible browser we intend to support, I could actually not find a claim of direct ES5 compatibility in our docs: https://nextjs.org/docs/basic-features/supported-browsers-features

[PizzaPete]

@balazsorban44 we're facing the same issue. Everything is compiled to ES5 but is-plain-object isn't and this is also causing issues for our project.

[balazsorban44]

Please open a separate bug report with a reproduction.

In this case, I would appreciate a minimal one with the source code, that is also deployed somewhere, so I can verify from an IE11 browser that it is actually happening.