💅 React TypeScript issue on The React variable is undeclared

[elawad]

Environment information

CLI:
  Version:                      1.9.1
  Color support:                true

Platform:
  CPU Architecture:             aarch64
  OS:                           macos

Environment:
  BIOME_LOG_PATH:               unset
  BIOME_LOG_PREFIX_NAME:        unset
  BIOME_CONFIG_PATH:            unset
  NO_COLOR:                     unset
  TERM:                         "xterm-256color"
  JS_RUNTIME_VERSION:           "v20.16.0"
  JS_RUNTIME_NAME:              "node"
  NODE_PACKAGE_MANAGER:         "yarn/1.22.22"

Biome Configuration:
  Status:                       Loaded successfully
  Formatter disabled:           false
  Linter disabled:              false
  Organize imports disabled:    false
  VCS disabled:                 false

Linter:
  JavaScript enabled:           true
  JSON enabled:                 true
  CSS enabled:                  true
  GraphQL enabled:              false
  Recommended:                  false
  All:                          false
  Enabled rules:
  performance/noDelete
  suspicious/noCatchAssign
  suspicious/noUnsafeNegation
  complexity/useLiteralKeys
  suspicious/noClassAssign
  style/useImportType
  complexity/noMultipleSpacesInRegularExpressionLiterals
  a11y/useValidLang
  complexity/noUselessEmptyExport
  suspicious/useNamespaceKeyword
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  correctness/noConstantCondition
  a11y/useAriaActivedescendantWithTabindex
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  style/useSelfClosingElements
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  correctness/noUnknownProperty
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  correctness/noUnusedLabels
  complexity/noUselessTernary
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  suspicious/noCompareNegZero
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  a11y/noAutofocus
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  style/noVar
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  correctness/noInvalidConstructorSuper
  suspicious/noImplicitAnyLet
  suspicious/noFallthroughSwitchClause
  suspicious/noUnsafeDeclarationMerging
  complexity/noUselessThisAlias
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  suspicious/noDuplicateObjectKeys
  complexity/noThisInStatic
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  correctness/noSelfAssign
  correctness/noInvalidBuiltinInstantiation
  style/noUselessElse
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  suspicious/noShadowRestrictedNames
  correctness/noInvalidDirectionInLinearGradient
  suspicious/noImportantInKeyframe
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  complexity/noUselessCatch
  correctness/noUnsafeFinally
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  complexity/noStaticOnlyClass
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  a11y/noInteractiveElementToNoninteractiveRole
  style/useNumericLiterals
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Rule name

noUndeclaredVariables

Playground link

https://biomejs.dev/playground/?lintRules=all&code=ZgB1AG4AYwB0AGkAbwBuACAATQB5AEEAcABwACgAKQAgAHsACgAgACAAZgB1AG4AYwB0AGkAbwBuACAAaABhAG4AZABsAGUAcgAoAGUAOgAgAFIAZQBhAGMAdAAuAEYAbwByAG0ARQB2AGUAbgB0ACkAIAB7AAoAIAAgACAAIAByAGUAdAB1AHIAbgAgAGUACgAgACAAfQAKAAoAIAAgAHIAZQB0AHUAcgBuACAAKAAKACAAIAAgACAAPABmAG8AcgBtACAAbwBuAFMAdQBiAG0AaQB0AD0AewAoAGUAKQAgAD0APgAgAGgAYQBuAGQAbABlAHIAKABlACkAfQA%2BAAoAIAAgACAAIAA8AC8AZgBvAHIAbQA%2BAAoAIAAgACkACgB9AAoACgBlAHgAcABvAHIAdAAgAHsAIABNAHkAQQBwAHAAIAB9AA%3D%3D

Expected result

Hello, we're migrating a few React 18/TS/EsLint projects into Biome. With React 18 you could use global React Types as React.JSX.Element, React.FormEvent, etc without an import statement.

But when we enable noUndeclaredVariables in Biome to check for possible bugs, we're getting a false positive on The React variable is undeclared.

A workaround is to explicitly add an import type React from 'react' on every file with React types. But we have hundreds of files that would need this. And it was working before migrating over.

Is there a better solution for this issue?

Thanks, and we love Biome!

Code of Conduct

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[Conaclos]

Hi! You can explicitly set a global using the javascript.globals configuration.

[elawad]

We did try globals, though that introduces other issues.

// biome.json
"javascript": {
  "globals": ["React"]
}

For instance, in a non-TS .jsx file you can add this without an import.

// App.jsx
function App() {
  const [count, setCount] = React.useState(0);
}

Biome will allow it due to globals, but the app will error during runtime.

One other option was just to disable noUndeclaredVariables in Biome overrides for *.tsx files.

[Conaclos]

Have you suggestions to solve the issue? We could add metadata to globals (if it is a type, a writable flobal, ...).

Also, we have a jsxRuntime configuration. i am unsure if we could leverage it in some way.

[arendjr]

I think the jsxRuntime should apply here indeed, but not sure if it does 😅

I’m also used to just disabling this rule, since it’s kinda redundant if you also use tsc.

[elawad]

I don't have any suggestions unfortunately. But disabling it seems to be ok, since TypeScript will handle undeclared issues on its own in .tsx files.

We do run biome check and tsc --noEmit in git hooks already. So that will find both types of issues.

[Conaclos]

This one of the reason noUndeclaredVariabels is still not recommended. It reports too much false positive. We hope to improve it once we have proper type info.

[elawad]

Thanks @Conaclos, it does help in .js/.jsx files. As we're used to it coming from ESLint.
But using overrides was the perfect solution here.

[Conaclos]

It seems that there are more false positives with types than variables. We could add an option to ignore undeclared types. We could allow the option by default and allow the rule by default in Biome 2.0.

What do you think?

[elawad]

That's a good idea. Since types are already checked by TypeScript tsc.

[marmikdesai]

I am facing same issue. Is there a plan to fix this?

[ematipico]

@marmikdesai please read the whole conversation, because it answers your question

[kurtextrem]

We could add an option to ignore undeclared types.

This would be awesome!

[Conaclos]

Actually, I think we should always ignore types in noUndeclaredVariables because the equivalent ESLint rule no-undef only check JS and TypeScript ESLint doesn't provide any extension for this rule.