TypeScript Target Compiler Option is Silently Ignored

[jasonboninger]

Verify canary release

• I verified that the issue exists in Next.js canary release

Provide environment information

Operating System:
Platform: win32
Arch: x64
Version: Windows 10 Home
Binaries:
Node: 16.13.0
npm: N/A
Yarn: N/A
pnpm: N/A
Relevant packages:
next: 12.1.7-canary.19
react: 18.1.0
react-dom: 18.1.0

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

next export

Describe the Bug

Changing the tsconfig.json -> compilerOptions -> target setting has no effect on static export builds and no error or warning occurs to indicate this.

This is my first time filing a bug report, so please let me know if I'm doing anything wrong or unhelpful!

Expected Behavior

Changing the TypeScript build target should be respected by the build engine like other TypeScript configuration settings, or an error or warning should indicate that the TypeScript target must always be a certain value.

To Reproduce

• Create a new project: npx create-next-app@canary --typescript
• Modify the build script in package.json to be next build && next export
  • Remove the Image import and Image tag from index.tsx because it will prevent static build from succeeding
• Modify pages/_app.tsx with a trivial piece of functionality which uses an ES2022 feature

import '../styles/globals.css'
import type { AppProps } from 'next/app'
import { useEffect, useState } from 'react';

class Test {
  #forMeOnly = true;
  
  doSomething(): void {
    console.log("FOR ME ONLY", this.#forMeOnly);
  }
}

function MyApp({ Component, pageProps }: AppProps) {
  const [test] = useState(new Test());
  useEffect(() => test.doSomething());
  return <Component {...pageProps} />
}

export default MyApp

• Modify compilerOptions -> target in tsconfig.json to be ES2022
• Execute npm run build
• Note that in the chunk exported for _app-<cache-bust>.js in out/_next/static/chunks/pages that the private field #forMeOnly is enforced using WeakMap and not using the native JavaScript feature from ES2022
• Expected to see #forMeOnly (or a minified version) and no WeakMap

[timneutkens]

Feel free to open a PR to add a warning. tsc is not used to compile the code so these options don't have an effect on output, only a few options that people were using babelrc for are respected.

[jasonboninger]

tsc is not used to compile the code so these options don't have an effect on output, only a few options that people were using babelrc for are respected.

Thanks for the quick reply! Is there another way to configure what JavaScript features the compiler considers as available? Perhaps the warning message could redirect people to that?

[balazsorban44]

We have an RFC to support Browserslist configuration: #33227

Experimental support was added in: #36584

[darshkpatel]

Feel free to open a PR to add a warning. tsc is not used to compile the code so these options don't have an effect on output, only a few options that people were using babelrc for are respected.

Faced this issue recently. I agree that adding a warning would be a good idea for now.
What warning message should be added @timneutkens @balazsorban44 ?
Happy to raise a PR.

[mkayander]

Hi everyone! Please feel free to share any thoughts on the linked PR i just pushed. #43582

[github-actions]

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