TypeScript (and JavaScript) Plugin

Rules for linting JavaScript and TypeScript code, including the latest and greatest powerful typed linting rules.
This plugin comes packaged with the flint npm package.

Presets

Flint’s TypeScript plugin provides the following presets:

Preset	Recommended	Description
logical	✅ Always	Common rules for finding bugs and enforcing good logical practices.
logicalStrict	☑️ When Ready	Additional rules for finding bugs and enforcing good logical practices.
stylistic	✅ Always	Common rules for consistent styling and best stylistic practices.
stylisticStrict	☑️ When Ready	Additional rules for consistent styling and best stylistic practices.
untyped		Extra rules for files that aren’t type-checked by TypeScript.

If you are just getting started with linting, Flint recommends using the logical and stylistic presets:

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: [ts.presets.logical, ts.presets.stylistic],
    },
  ],
});

If you are experienced with both JavaScript/TypeScript and linting, Flint recommends using the logicalStrict and stylisticStrict presets:

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: [ts.presets.logicalStrict, ts.presets.stylisticStrict],
    },
  ],
});

Note

Coming soon: more expansive documentation on choosing and combining plugins & presets with Flint.

logical

Rules that find bugs and enforce good logical practices for most-to-all JavaScript and TypeScript files.

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: ts.presets.logical,
    },
  ],
});

logicalStrict

Additional logical rules that enforce best practices which are not always straightforward to implement. These rules are recommended for projects where a majority of developers are experienced with both JavaScript/TypeScript and using a linter.

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: ts.presets.logicalStrict,
    },
  ],
});

This preset’s rules are a superset of those in logical.

stylistic

Rules that enforce consistent styling and best stylistic practices for most-to-all JavaScript and TypeScript files.

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: ts.presets.stylistic,
    },
  ],
});

stylisticStrict

Additional stylistic rules that enforce best practices which are not always straightforward to implement. These rules are recommended for projects where a majority of developers are experienced with both JavaScript/TypeScript and using a linter.

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: ts.presets.stylisticStrict,
    },
  ],
});

This preset’s rules are a superset of those in stylistic.

untyped

Rules that fill in rudimentary safety practices that would normally be caught by TypeScript.

flint.config.ts

import { defineConfig, ts } from "flint";

export default defineConfig({
  use: [
    {
      files: ts.files.all,
      rules: ts.presets.untyped,
    },
  ],
});

The untyped preset is broadly applicable for JavaScript files that aren’t type-checked by TypeScript. For long-lived projects, Flint recommends using this only as a stopgap measure pending converting files fully to TypeScript.

Rules

Implemented: 57 of 399 (14.2%)

Flint Rule	Preset
anyArguments	logical
anyAssignments	logical
anyCalls	logical
anyMemberAccess	logical
anyReturnsReports returning a value with type any from a function.	logical
arguments	logical
arrayConstructors	logical
arrayElementDeletions	logical
arrayEmptyCallbackSlots	logical
arrayMapIdentities	logical
arrayUnnecessaryLengthChecks	logical
asyncFunctionAwaits	logical
asyncPromiseExecutorsReports using async functions as Promise executor functions.	logical
asyncUnnecessaryPromiseWrappers	logical
awaitThenable	logical
caseDuplicatesReports switch statements with duplicate case clause test expressions.	logical
caseFallthroughs	logical
catchCallbackTypes	logical
charAtComparisons	logical
constVariables	logical
dateConstructorClones	logical
debuggerStatementsReports using debugger statements.	logical
defaultCaseLastReports switch statements where the default clause is not last.	logical
deprecated	logical
dynamicDeletes	logical
elseIfDuplicatesReports duplicate conditions in if-else-if chains that make code unreachable.	logical
emptyDestructuresReports using empty destructuring patterns that destructure no values.	logical
emptyEnums	logical
emptyExports	logical
emptyObjectTypes	logical
enumMemberLiterals	logical
enumMixedValues	logical
enumValueConsistency	logical
enumValueDuplicates	logical
equalityOperators	logical
errorUnnecessaryCaptureStackTraces	logical
evals	logical
exceptionAssignmentsReports reassigning exception parameters in catch clauses.	logical
explicitAnys	logical
exportMutables	logical
fetchMethodBodies	logical
finallyStatementSafetyReports control flow statements in finally blocks that can override control flow in try/catch blocks.	logical
floatingPromises	logical
forInArraysReports iterating over an array with a for-in loop.	logical
functionCurryingRedundancyReports using .apply() or .call() or when the context (this value) provides no benefit.	logical
functionNewCallsReports using the Function constructor to create functions from strings.	logical
generatorFunctionYieldsReports generator functions that do not yield values.	logical
getterSetterPairedTypes	logical
impliedEvals	logical
importEmptyBlocks	logical
instanceOfArrays	logical
irregularWhitespaces	logical
isNaNComparisons	logical
meaninglessVoidOperators	logical
misleadingVoidExpressions	logical
misusedPromises	logical
moduleSpecifierLists	logical
namespaceDeclarationsReports using legacy namespace declarations.	logical
negativeZeroComparisonsReports comparisons with -0 that may not behave as expected.	logical
newDefinitions	logical
newExpressionsReports standalone new expressions that don't use the constructed object.	logical
nonNullAssertedOptionalChains	logical
nonOctalDecimalEscapesReports non-octal decimal escape sequences (\8 and \9) in string literals.	logical
numberMethodRanges	logical
numericErasingOperations	logical
numericPrecision	logical
objectCallsPrefer {} object literal notation or Object.create instead of calling or constructing Object.	logical
objectPrototypeBuiltInsReports direct calls to Object.prototype methods on object instances.	logical
objectSpreadUnnecessaryFallbacks	logical
parameterPropertyAssignment	logical
parseIntRadixes	logical
plusOperands	logical
promiseExecutorReturns	logical
promiseFinallyReturns	logical
promiseMethodSingleArrayArguments	logical
promiseRejectErrors	logical
recursionOnlyArguments	logical
redundantTypeConstituents	logical
regexAllGlobalFlags	logical
regexAmbiguousInvalidity	logical
regexContradictoryAssertions	logical
regexControlCharacterEscapes	logical
regexControlCharacters	logical
regexDuplicateCharacterClassCharacters	logical
regexDuplicateDisjunctions	logical
regexEmptyAlternatives	logical
regexEmptyCapturingGroups	logical
regexEmptyCharacterClasses	logical
regexEmptyGroups	logical
regexEmptyLazyQuantifiers	logical
regexEmptyLookaroundsAssertions	logical
regexEmptyStringLiterals	logical
regexEscapeBackspaces	logical
regexIgnoreCaseFlags	logical
regexInvisibleCharacters	logical
regexLegacyFeatures	logical
regexLiterals	logical
regexLookaroundQuantifierOptimizations	logical
regexMisleadingCapturingGroups	logical
regexMisleadingQuantifiers	logical
regexMisleadingUnicodeCharacters	logical
regexNamedCaptureGroups	logical
regexNonStandardFlags	logical
regexObscureRanges	logical
regexOctalEscapes	logical
regexQuantifierOptimizations	logical
regexSetOperationOptimizations	logical
regexStandaloneBackslashes	logical
regexSuperLinearBacktracking	logical
regexSuperLinearMoves	logical
regexUnnecessaryAssertions	logical
regexUnnecessaryBackreferences	logical
regexUnnecessaryCharacterClasses	logical
regexUnnecessaryCharacterRanges	logical
regexUnnecessaryDisjunctions	logical
regexUnnecessaryDollarReplacements	logical
regexUnnecessaryLookaroundAssertions	logical
regexUnnecessaryNestedAssertions	logical
regexUnnecessaryNestedQuantifiers	logical
regexUnnecessaryNumericQuantifiers	logical
regexUnnecessaryOptionalAssertions	logical
regexUnnecessaryReferentialBackreferences	logical
regexUnnecessarySetOperands	logical
regexUnusedCapturingGroups	logical
regexUnusedFlags	logical
regexUnusedLazyQuantifiers	logical
regexUnusedQuantifiers	logical
regexValidity	logical
regexZeroQuantifiers	logical
requireImports	logical
returnAwaitPromises	logical
selfAssignmentsReports self-assignments which have no effect and are likely errors.	logical
singleVariableDeclarations	logical
sparseArraysReports array literals with holes (sparse arrays).	logical
strictBooleanExpressions	logical
stringCaseMismatches	logical
templateExpressionValues	logical
thisAliases	logical
throwErrors	logical
tripleSlashReferences	logical
tripleSlashReferenceValidity	logical
tsComments	logical
typeConstituentDuplicates	logical
unboundMethods	logical
unnecessaryBind	logical
unnecessaryCatchesReports catch clauses that only rethrow the caught error without modification.	logical
unnecessaryComparisons	logical
unnecessaryConditions	logical
unnecessaryContinues	logical
unnecessaryLogicalComparisons	logical
unnecessaryMathClamps	logical
unnecessaryNumericFractions	logical
unnecessarySpreads	logical
unnecessaryTemplateExpressions	logical
unnecessaryTypeArguments	logical
unnecessaryTypeAssertions	logical
unnecessaryTypeConstraints	logical
unnecessaryTypeConversions	logical
unnecessaryTypeParameters	logical
unnecessaryUndefinedDefaults	logical
unnecessaryUseStricts	logical
unsafeDeclarationmerging	logical
unsafeEnumComparisons	logical
unsafeFunctionTypes	logical
unsafeToString	logical
unsafeTypeAssertions	logical
unsafeUnaryNegations	logical
unusedExpressions	logical
unusedPrivateClassMembers	logical
unusedSwitchStatements	logical
unusedValues	logical
unusedVariables	logical
uselessDefaultAssignments	logical
wrapperObjects	logical
wrapperObjectTypes	logical
accessorThisRecursion	logical (strict)
awaitInsidePromiseMethods	logical (strict)
caughtErrorCauses	logical (strict)
dateNowTimestamps	logical (strict)
directivePairs	logical (strict)
errorMessages	logical (strict)
errorSubclassProperties	logical (strict)
extraneousClasses	logical (strict)
importExtraneousDependencies	logical (strict)
invalidVoidTypes	logical (strict)
nonNullAssertedNullishCoalesces	logical (strict)
nonNullAssertions	logical (strict)
reduceTypeParameters	logical (strict)
regexGraphemeStringLiterals	logical (strict)
returnThisTypes	logical (strict)
selfComparisonsReports comparing a value to itself.	logical (strict)
stringCodePoints	logical (strict)
unifiedSignatures	logical (strict)
arrayDeleteUnnecessaryCounts	stylistic
arrayExistenceChecksConsistency	stylistic
arrayFinds	stylistic
arrayFlatUnnecessaryDepths	stylistic
arrayIncludes	stylistic
arrayLoops	stylistic
arrayMutableReverses	stylistic
arrayMutableSorts	stylistic
arraySliceUnnecessaryEnd	stylistic
arrayTernarySpreadingConsistency	stylistic
arrayTypes	stylistic
asConstAssertions	stylistic
assignmentOperatorShorthands	stylistic
builtinConstructorNews	stylistic
chainedAssignmentsReports using chained assignment expressions (e.g., a = b = c).	stylistic
classLiteralProperties	stylistic
consecutiveNonNullAssertionsReports unnecessary extra non-null assertions.	stylistic
emptyBlocksReports empty block statements that should contain code.	stylistic
emptyModuleAttributes	stylistic
emptyStaticBlocksReports empty static initialization blocks within class declarations.	stylistic
emptyTypeParameterLists	stylistic
exponentiationOperators	stylistic
exportFromImports	stylistic
forDirectionsReports for loops with counter variables that move in the wrong direction.	stylistic
functionCallSpreads	stylistic
functionTypeDeclarations	stylistic
genericConstructorCalls	stylistic
groupedAccessorPairs	stylistic
importCycles	stylistic
importDuplicates	stylistic
importSelf	stylistic
importUnnecessaryPathSegments	stylistic
indexedObjectTypes	stylistic
jsdocAccessTags	stylistic
jsdocEmptyBlocks	stylistic
jsdocEmptyTags	stylistic
jsdocImplementsTags	stylistic
jsdocParameterNames	stylistic
jsdocPropertyNames	stylistic
jsdocRedundantTypes	stylistic
jsdocTemplateNames	stylistic
jsdocTypesSyntax	stylistic
jsdocUnnecessaryReturns	stylistic
jsdocUnnecessaryYields	stylistic
jsdocValidTypes	stylistic
jsdocValues	stylistic
jsdocYields	stylistic
literalConstructorWrappers	stylistic
methodSignatureStyles	stylistic
multilineAmbiguities	stylistic
namespaceKeywords	stylistic
nestedStandaloneIfs	stylistic
nonNullableTypeAssertions	stylistic
nullishCoalescingOperators	stylistic
numericLiteralParsingReports parseInt calls with binary, hexadecimal, or octal strings that can be replaced with numeric literals.	stylistic
objectAssignSpreads	stylistic
objectHasOwnsPrefer Object.hasOwn() over Object.prototype.hasOwnProperty.call() for checking own properties.	stylistic
objectShorthand	stylistic
objectTypeDefinitions	stylistic
operatorAssignmentShorthand	stylistic
optionalChainOperators	stylistic
overloadSignaturesAdjacent	stylistic
promiseFunctionAsync	stylistic
propertyAccessNotation	stylistic
regexCharacterClassRanges	stylistic
regexCharacterClassSetOperations	stylistic
regexConciseCharacterClassNegations	stylistic
regexDollarEscapes	stylistic
regexPredefinedAssertions	stylistic
regexRepeatQuantifiers	stylistic
regexTestMethods	stylistic
regexUnicodeEscapes	stylistic
regexUnicodeProperties	stylistic
regexUnnecessaryEscapes	stylistic
responseMethods	stylistic
returnAssignmentsReports using assignment expressions in return statements.	stylistic
shadows	stylistic
stringStartsEndsWith	stylistic
symbolDescriptionsReports Symbol() calls without description arguments.	stylistic
tslintComments	stylistic
typeAssertions	stylistic
typeExports	stylistic
typeImports	stylistic
undefinedInitialValues	stylistic
unicodeBOMsReports files with Unicode Byte Order Marks (BOMs).	stylistic
unnecessaryBlocksReports standalone block statements that don't create a meaningful scope.	stylistic
unnecessaryBooleanCasts	stylistic
unnecessaryComputedKeys	stylistic
unnecessaryConcatenationReports string concatenation using the + operator when both operands are string literals.	stylistic
unnecessaryConstructors	stylistic
unnecessaryEscapes	stylistic
unnecessaryQualifiers	stylistic
unnecessaryRenames	stylistic
unnecessaryReturns	stylistic
unnecessaryTypeAnnotations	stylistic
unusedLabels	stylistic
varDeclarations	stylistic
voidOperatorReports using the void operator.	stylistic
arrayFilteredFinds	stylistic (strict)
arrayFlatMapMethods	stylistic (strict)
arrayFlatMethods	stylistic (strict)
arrayIncludesMethods	stylistic (strict)
arrayIndexOfMethods	stylistic (strict)
arraySomeMethods	stylistic (strict)
atAccesses	stylistic (strict)
builtinCoercions	stylistic (strict)
caughtVariableNames	stylistic (strict)
classMethodsThis	stylistic (strict)
combinedPushes	stylistic (strict)
destructuringConsistency	stylistic (strict)
directiveRequireDescriptions	stylistic (strict)
elseReturns	stylistic (strict)
emptyFiles	stylistic (strict)
emptyFunctions	stylistic (strict)
escapeSequenceCasing	stylistic (strict)
functionDefinitionScopeConsistency	stylistic (strict)
globalThisAliases	stylistic (strict)
jsdocAsterisks	stylistic (strict)
jsdocInformativeDocs	stylistic (strict)
jsdocMisleadingBlocks	stylistic (strict)
jsdocMultilineBlocks	stylistic (strict)
jsdocParameterDescriptionHyphens	stylistic (strict)
jsdocTagNames	stylistic (strict)
mathMethods	stylistic (strict)
namedDefaultExports	stylistic (strict)
namespaceImplicitAmbientImports	stylistic (strict)
negativeIndexLengthMethods	stylistic (strict)
nonNullAssertionPlacement	stylistic (strict)
numberStaticMethods	stylistic (strict)
numericLiteralCasing	stylistic (strict)
numericSeparatorGroups	stylistic (strict)
objectEntriesMethods	stylistic (strict)
parameterReassignments	stylistic (strict)
regexCharacterClasses	stylistic (strict)
regexDigitMatchers	stylistic (strict)
regexExecutors	stylistic (strict)
regexHexadecimalEscapes	stylistic (strict)
regexLetterCasing	stylistic (strict)
regexLookaroundAssertions	stylistic (strict)
regexMatchNotation	stylistic (strict)
regexNamedBackreferences	stylistic (strict)
regexNamedReplacements	stylistic (strict)
regexPlusQuantifiers	stylistic (strict)
regexQuestionQuantifiers	stylistic (strict)
regexResultArrayGroups	stylistic (strict)
regexStarQuantifiers	stylistic (strict)
regexUnicodeCodepointEscapes	stylistic (strict)
regexUnnecessaryNonCapturingGroups	stylistic (strict)
regexWordMatchers	stylistic (strict)
setHasExistenceChecks	stylistic (strict)
setSizeLengthChecks	stylistic (strict)
sizeComparisonOperators	stylistic (strict)
staticMemberOnlyClasses	stylistic (strict)
stringSliceMethods	stylistic (strict)
stringTrimMethods	stylistic (strict)
structuredCloneMethods	stylistic (strict)
topLevelAwaits	stylistic (strict)
undefinedTypeofChecks	stylistic (strict)
unnecessaryTernaries	stylistic (strict)
arrayCallbackReturns	untyped
caseDeclarationsReports lexical declarations in case clauses without wrapping them in blocks.	untyped
classAssignmentsReports reassigning class declarations.	untyped
classFieldDeclarations	untyped
classMemberDuplicates	untyped
constantAssignmentsReports attempting to reassign variables declared with const.	untyped
constructorReturnsReports returning values from constructor functions.	untyped
constructorSupers	untyped
defaultParameterLast	untyped
duplicateArgumentsReports functions with duplicate parameter names in their signatures.	untyped
functionAssignmentsReports reassigning variables declared with function declarations.	untyped
getterReturns	untyped
globalAssignmentsReports attempting to assign to read-only global variables such as undefined, NaN, Infinity, Object, etc.	untyped
globalObjectCallsReports calling global objects like Math, JSON, or Reflect as functions.	untyped
importAssignments	untyped
invalidThis	untyped
nativeObjectExtensions	untyped
newNativeNonConstructorsDisallows using new with global non-constructor functions like Symbol and BigInt.	untyped
objectKeyDuplicatesReports unnecessary duplicate keys that override previous values.	untyped
objectProtoReports using the deprecated proto property to access or modify an object's prototype.	untyped
octalEscapesReports using octal escape sequences in string literals.	untyped
octalNumbersReports using legacy octal numeric literals.	untyped
sequencesReports using the comma operator in expressions.	untyped
setterReturns	untyped
shadowedRestrictedNamesReports variable declarations that shadow JavaScript's restricted names.	untyped
thisBeforeSuper	untyped
typeofComparisonsReports typeof expressions that compare impossible string literals.	untyped
unassignedVariablesReports variables that are declared but never assigned a value.	untyped
undefinedVariablesReports using variables that are not defined.	untyped
unreachableStatements	untyped
unsafeNegationsReports negating the left operand of in or instanceof relations.	untyped
unsafeOptionalChains	untyped
usageBeforeDefinition	untyped
useStrictDirectives	untyped
variableBlockScopeUsage	untyped
variableDeletionsReports attempting to delete variables with the delete operator.	untyped
variableRedeclarations	untyped
withStatementsReports using with statements	untyped
consoleCalls	(none)
importTypeSideEffects	(none)
regexUnicodeFlag	(none)
restrictedGlobals	(none)
restrictedIdentifiers	(none)
restrictedImports	(none)
restrictedProperties	(none)
restrictedSyntax	(none)
restrictedTypes	(none)

Selectors

Flint’s TypeScript plugin provides the following file selector:

• all: **/*.{cjs,js,jsx,mjs,ts,tsx}