💅 Transform a function with generics to arrow function by biome linter will not safe.

[MTRo]

Environment information

CLI:
  Version:                      1.8.2
  Color support:                true

Platform:
  CPU Architecture:             x86_64
  OS:                           linux

Environment:
  BIOME_LOG_DIR:                unset
  NO_COLOR:                     unset
  TERM:                         "xterm-256color"
  JS_RUNTIME_VERSION:           "v20.15.0"
  JS_RUNTIME_NAME:              "node"
  NODE_PACKAGE_MANAGER:         "npm/10.7.0"

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

Linter:
  JavaScript enabled:           true
  JSON enabled:                 true
  CSS enabled:                  false
  Recommended:                  true
  All:                          false
  Enabled rules:
  performance/noDelete
  suspicious/noCatchAssign
  suspicious/noUnsafeNegation
  complexity/useLiteralKeys
  complexity/noMultipleSpacesInRegularExpressionLiterals
  a11y/useValidLang
  complexity/noUselessEmptyExport
  suspicious/useNamespaceKeyword
  suspicious/useValidTypeof
  a11y/useValidAriaRole
  correctness/noConstantCondition
  a11y/useAriaActivedescendantWithTabindex
  suspicious/noAssignInExpressions
  style/useDefaultParameterLast
  complexity/noEmptyTypeParameters
  correctness/noConstructorReturn
  style/useSelfClosingElements
  suspicious/noDuplicateParameters
  style/useTemplate
  correctness/noUnusedLabels
  complexity/noUselessTernary
  correctness/noUnreachableSuper
  suspicious/noCompareNegZero
  suspicious/noExplicitAny
  correctness/noSwitchDeclarations
  a11y/noAutofocus
  correctness/noUnsafeOptionalChaining
  correctness/noConstAssign
  suspicious/noControlCharactersInRegex
  complexity/noUselessTypeConstraint
  style/noVar
  suspicious/noDoubleEquals
  suspicious/noRedundantUseStrict
  style/useLiteralEnumMembers
  suspicious/noGlobalIsNan
  suspicious/noEmptyInterface
  suspicious/noConstEnum
  suspicious/noMisleadingCharacterClass
  correctness/noPrecisionLoss
  suspicious/noRedeclare
  correctness/noStringCaseMismatch
  correctness/noSetterReturn
  correctness/noInvalidConstructorSuper
  suspicious/noImplicitAnyLet
  suspicious/noFallthroughSwitchClause
  suspicious/noUnsafeDeclarationMerging
  correctness/noUnreachable
  a11y/useKeyWithClickEvents
  suspicious/noDuplicateObjectKeys
  complexity/noUselessThisAlias
  complexity/noThisInStatic
  complexity/useOptionalChain
  correctness/noInnerDeclarations
  style/noParameterAssign
  suspicious/noDuplicateCase
  a11y/useValidAnchor
  complexity/useRegexLiterals
  correctness/noSelfAssign
  style/noUselessElse
  style/useShorthandFunctionType
  suspicious/noShadowRestrictedNames
  a11y/useMediaCaption
  complexity/noUselessLabel
  complexity/noUselessCatch
  correctness/noUnsafeFinally
  a11y/useAriaPropsForRole
  correctness/noNonoctalDecimalEscape
  style/useEnumInitializers
  a11y/useHtmlLang
  suspicious/noDuplicateTestHooks
  complexity/noStaticOnlyClass
  style/useWhile
  complexity/useArrowFunction
  style/noInferrableTypes
  a11y/noNoninteractiveTabindex
  complexity/useSimpleNumberKeys
  correctness/useYield
  a11y/noInteractiveElementToNoninteractiveRole
  style/useNumericLiterals
  correctness/noUnnecessaryContinue
  suspicious/noApproximativeNumericConstant
  suspicious/noImportAssign
  suspicious/noLabelVar
  correctness/noGlobalObjectCalls
  suspicious/useDefaultSwitchClauseLast
  a11y/useAltText
  correctness/noEmptyCharacterClassInRegex
  suspicious/noSuspiciousSemicolonInJsx
  suspicious/noSparseArray
  a11y/useIframeTitle
  complexity/noBannedTypes
  a11y/noSvgWithoutTitle
  correctness/noVoidElementsWithChildren
  style/useAsConstAssertion
  correctness/useJsxKeyInIterable
  style/useExportType
  complexity/noUselessLoneBlockStatements
  suspicious/noPrototypeBuiltins
  suspicious/noMisleadingInstantiator
  suspicious/noDebugger
  style/noArguments
  a11y/useValidAriaValues
  suspicious/noCommentText
  suspicious/noThenProperty
  suspicious/noDuplicateJsxProps
  suspicious/noGlobalAssign
  a11y/noPositiveTabindex
  correctness/noEmptyPattern
  complexity/noExcessiveNestedTestSuites
  security/noDangerouslySetInnerHtmlWithChildren
  a11y/useKeyWithMouseEvents
  suspicious/noExtraNonNullAssertion
  suspicious/useGetterReturn
  correctness/noRenderReturnValue
  correctness/useExhaustiveDependencies
  security/noGlobalEval
  style/noNonNullAssertion
  a11y/noRedundantRoles
  complexity/useFlatMap
  correctness/useIsNan
  correctness/noUnusedVariables
  style/useConst
  suspicious/noGlobalIsFinite
  suspicious/noSelfCompare
  suspicious/noAsyncPromiseExecutor
  security/noDangerouslySetInnerHtml
  style/useNodejsImportProtocol
  a11y/noDistractingElements
  suspicious/noArrayIndexKey
  complexity/noWith
  suspicious/noDuplicateClassMembers
  complexity/noExtraBooleanCast
  performance/noAccumulatingSpread
  a11y/useValidAriaProps
  a11y/noRedundantAlt
  correctness/noChildrenProp
  suspicious/noConfusingLabels
  suspicious/noConfusingVoidType
  suspicious/noFocusedTests
  a11y/useButtonType
  a11y/noAriaUnsupportedElements
  correctness/noFlatMapIdentity
  a11y/noBlankTarget
  a11y/useHeadingContent
  correctness/useValidForDirection
  correctness/noVoidTypeReturn
  correctness/noInvalidUseBeforeDeclaration
  a11y/noAriaHiddenOnFocusable
  a11y/useAnchorContent
  complexity/noUselessRename
  correctness/noInvalidNewBuiltin
  style/useNumberNamespace
  complexity/noUselessConstructor
  a11y/noAccessKey
  style/useExponentiationOperator
  style/noUnusedTemplateLiteral
  complexity/noUselessSwitchCase
  style/useSingleVarDeclarator
  suspicious/noExportsInTest
  a11y/noNoninteractiveElementToInteractiveRole
  style/noCommaOperator
  suspicious/useIsArray
  a11y/noHeaderScope
  complexity/noUselessFragments
  suspicious/noMisrefactoredShorthandAssign
  complexity/noForEach
  correctness/noUnusedImports
  suspicious/noClassAssign
  suspicious/noFunctionAssign

Workspace:
  Open Documents:               0

Rule name

lint/complexity/useArrowFunction

Playground link

https://biomejs.dev/playground/?code=CgAKAGMAbwBuAHMAdAAgAHcAaQB0AGgARABlAGYAYQB1AGwAdAAgAD0AIABmAHUAbgBjAHQAaQBvAG4AIAA8AFQAPgAoAHYAYQBsAHUAZQA6ACAAVAAsACAAZABlAGYAYQB1AGwAdABWAGEAbAB1AGUAOgAgAFQAKQAgAHsACgAgACAAcgBlAHQAdQByAG4AIAB2AGEAbAB1AGUAIAA%2FAD8AIABkAGUAZgBhAHUAbAB0AFYAYQBsAHUAZQA7AAoAfQA7AAoACgAvAC8AIABjAG8AbgBzAHQAIAB3AGkAdABoAEQAZQBmAGEAdQBsAHQAIAA9ACAAPABUAD4AKAB2AGEAbAB1AGUAOgAgAFQALAAgAGQAZQBmAGEAdQBsAHQAVgBhAGwAdQBlADoAIABUACkAIAA9AD4AIAB2AGEAbAB1AGUAIAA%2FAD8AIABkAGUAZgBhAHUAbAB0AFYAYQBsAHUAZQA7AAoA

Expected result

Hi there, I discovered some weird behavior with Biome linter, so I want to share it.

When I apply to biome check --write to this code below (please don't mind that the functions actual doing.)

const withDefault = function <T>(value: T, defaultValue: T) {
    return value ?? defaultValue;
};

Biome linter suggest (also will be replaced) to this

const withDefault = <T>(value: T, defaultValue: T) => value ?? defaultValue;

However, unfortunately it seems probably wrong. And I actually expect to replace like this ( <T,> )

const withDefault = <T,>(value: T, defaultValue: T) => value ?? defaultValue;

Here is Typescript playground, and you can see how typescript recognize those code.

https://www.typescriptlang.org/play/?#code/FAehAIGMHsDsGcAu4DuBLRALAIgUwGYCGArgDbIC84+xskiac4APACoB8AFAG6GnG4AXOFYAacABMCJcgDU+A4awCU4AN6gI4beABOuRMV2xwvfrnAB+S5OllE88wG5N4AL4vgMBMnRY8RPYAqrC4AB4ADrj0uBLgVGyiXGaKIuJSgXIKQiKqFOym2VY2GTIO2Z7eSKgYOHbkAKKR0Yix8SwcPNlK6fXl5kp5BSkW1raZ-QIuQA

And also this post said what should suggest to.

https://stackoverflow.com/questions/32308370/what-is-the-syntax-for-typescript-arrow-functions-with-generics

Thanks!

Code of Conduct

• I agree to follow Biome's Code of Conduct

[Sec-ant]

const withDefault = <T>(value: T, defaultValue: T) => value ?? defaultValue; is correct syntax if JSX is disabled: https://www.typescriptlang.org/play/?jsx=0#code/FAehAIGMHsDsGcAu4DuBLRALAIgUwGYCGArgDbIC84+xskiac4APACoB8AFAG6GnG4AXOFYAacABMCJcgDU+A4awCU4AN6gI4beABOuRMV2xwvfrnAB+S5OllE88wG5N4AL4vgMBMnRY8RPYAqrC4AB4ADrj0uBLgVGyiXGaKIuJSgXIKQiKqFOym2VY2GTIO2Z7eSKgYOHbkAKKR0Yix8SwcPNlK6fXl5kp5BSkW1raZ-QIuQA

I see two options:

1. Check the file extension (.ts or .tsx) to decide whether to use <T> or <T,>.
2. Always use <T,> because our formatter can format it into <T> if it is a TypeScript file with JSX disabled: https://biomejs.dev/playground/?code=YwBvAG4AcwB0ACAAdwBpAHQAaABEAGUAZgBhAHUAbAB0ACAAPQAgADwAVAAsAD4AKAB2AGEAbAB1AGUAOgAgAFQALAAgAGQAZQBmAGEAdQBsAHQAVgBhAGwAdQBlADoAIABUACkAIAA9AD4AIAB2AGEAbAB1AGUAIAA%2FAD8AIABkAGUAZgBhAHUAbAB0AFYAYQBsAHUAZQA7AA%3D%3D&jsx=false

---

I think we should go with option 1 because JSX can only appear in an .tsx file.

[MTRo]

Sorry my late response. It sounds good idea!
Because change always to <T,> will bit confuse someone (like me)
I still have no idea why I need weird comma :p