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# Linting
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Rome's linter provides static analysis and code quality checks that replace ESLint, with extensive rules for correctness, performance, accessibility, security, and style. It includes automatic fixing capabilities for many issues.
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## Capabilities
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### Check Command
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The primary linting command that analyzes files for issues and optionally applies fixes.
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```bash { .api }
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/**
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 * Run the linter on a set of files
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 * 
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 * Usage: rome check [OPTIONS] <INPUTS...>
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 *
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 * INPUTS can be one or more filesystem paths, each pointing to a single file
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 * or an entire directory to be searched recursively for supported files
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 */
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rome check <INPUTS...>
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```
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**Core Options:**
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```bash { .api }
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--apply                    # Apply safe fixes automatically
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--apply-suggested         # Apply safe and suggested fixes automatically  
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--max-diagnostics <number> # Cap the amount of diagnostics displayed (default: 20)
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--verbose                 # Print additional verbose advice on diagnostics
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```
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**Usage Examples:**
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```bash
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# Check files without fixing
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rome check src/
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# Apply safe fixes only
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rome check src/ --apply
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# Apply both safe and suggested fixes
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rome check src/ --apply-suggested
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# Limit diagnostic output
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rome check src/ --max-diagnostics=10
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# Verbose diagnostic information
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rome check src/ --verbose
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```
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### Fix Application Types
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Rome categorizes fixes into different safety levels:
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```bash { .api }
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# Safe fixes: Low-risk changes that preserve code behavior
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--apply
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# Suggested fixes: Higher-risk changes that may alter behavior  
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--apply-suggested
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```
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## Rule Categories
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### Accessibility Rules (A11y)
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Rules that enforce web accessibility best practices.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "a11y": {
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        "recommended": true,
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        "noAutofocus": "error",        // Avoid autoFocus attribute
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        "noBlankTarget": "error",      // Disallow target="_blank" without rel="noreferrer"
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        "noPositiveTabindex": "error", // Prevent positive tabIndex values
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        "useAltText": "error",         // Require alt text for images
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        "useAnchorContent": "error",   // Enforce anchor element content
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        "useButtonType": "error",      // Enforce button type attribute
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        "useKeyWithClickEvents": "error", // Require keyboard events with click events
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        "useKeyWithMouseEvents": "error"  // Require keyboard events with mouse events
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      }
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    }
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  }
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}
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```
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### Complexity Rules
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Rules that enforce code complexity limits and maintainability.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "complexity": {
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        "recommended": true,
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        "noBannedTypes": "error",      // Disallow certain built-in types
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        "noExtraBooleanCast": "error", // Avoid unnecessary boolean casts
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        "noMultipleSpacesInRegularExpressionLiterals": "error",
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        "noStaticOnlyClass": "error",  // Disallow classes with only static members
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        "noUselessCatch": "error",     // Disallow unnecessary catch clauses
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        "noUselessConstructor": "error", // Disallow unnecessary constructors
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        "noUselessEmptyExport": "error", // Disallow empty export statements
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        "noUselessFragments": "error",   // Disallow unnecessary React fragments
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        "noUselessLabel": "error",       // Disallow unused labels
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        "noUselessRename": "error",      // Disallow renaming import/export to same name
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        "noUselessSwitchCase": "error",  // Disallow useless switch cases
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        "noWith": "error",               // Disallow with statements
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        "useLiteralKeys": "error",       // Enforce literal keys in objects
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        "useSimplifiedLogicExpression": "error" // Simplify boolean expressions
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      }
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    }
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  }
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}
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```
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### Correctness Rules
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Rules that catch potential bugs and logical errors.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "recommended": true,
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        "noChildrenProp": "error",         // Disallow children prop in React
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        "noConstAssign": "error",          // Disallow reassigning const variables
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        "noConstantCondition": "error",    // Disallow constant conditions
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        "noEmptyCharacterClassInRegex": "error", // Disallow empty character classes
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        "noEmptyPattern": "error",         // Disallow empty destructuring patterns
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        "noGlobalObjectCalls": "error",    // Disallow calling global objects as functions
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        "noInnerDeclarations": "error",    // Disallow variable/function declarations in nested blocks
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        "noInvalidConstructorSuper": "error", // Disallow invalid super() calls
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        "noNewSymbol": "error",            // Disallow new Symbol()
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        "noPrecisionLoss": "error",        // Disallow numeric literals that lose precision
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        "noSelfAssign": "error",           // Disallow self assignment
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        "noSetterReturn": "error",         // Disallow return in setter
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        "noSwitchDeclarations": "error",   // Disallow lexical declarations in switch clauses
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        "noUndeclaredVariables": "error",  // Disallow undeclared variables
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        "noUnreachable": "error",          // Disallow unreachable code
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        "noUnreachableSuper": "error",     // Disallow unreachable super() calls
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        "noUnsafeFinally": "error",        // Disallow unsafe control flow in finally blocks
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        "noUnusedLabels": "error",         // Disallow unused labels
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        "noUnusedVariables": "error",      // Disallow unused variables
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        "useIsNan": "error",               // Require isNaN() when checking for NaN
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        "useValidForDirection": "error",   // Enforce correct for loop directions
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        "useYield": "error"                // Require yield in generator functions
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      }
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    }
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  }
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}
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```
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### Performance Rules
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Rules that identify potential performance issues.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "performance": {
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        "recommended": true,
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        "noDelete": "error"               // Disallow delete operator
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      }
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    }
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  }
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}
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```
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### Security Rules
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Rules that help prevent security vulnerabilities.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "security": {
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        "recommended": true,
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        "noDangerouslySetInnerHtml": "error", // Disallow dangerouslySetInnerHTML
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        "noGlobalEval": "error"               // Disallow eval() in global scope
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      }
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    }
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  }
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}
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```
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### Style Rules
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Rules that enforce consistent code style.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "style": {
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        "recommended": true,
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        "noArguments": "error",            // Disallow arguments object
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        "noCommaOperator": "error",        // Disallow comma operator
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        "noImplicitBoolean": "error",      // Disallow implicit boolean conversion
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        "noInferrableTypes": "error",      // Disallow obvious type annotations
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        "noNamespace": "error",            // Disallow TypeScript namespaces
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        "noNonNullAssertion": "error",     // Disallow non-null assertions
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        "noParameterAssign": "error",      // Disallow reassigning function parameters
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        "noParameterProperties": "error",  // Disallow parameter properties
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        "noRestrictedGlobals": "error",    // Disallow specific global variables
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        "noShoutyConstants": "error",      // Disallow SHOUTY_CASE constants
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        "noUnusedTemplateLiteral": "error", // Disallow unused template literals
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        "noVar": "error",                  // Disallow var declarations
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        "useBlockStatements": "error",     // Enforce block statements
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        "useCollapsedElseIf": "error",     // Enforce collapsed else-if
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        "useConst": "error",               // Enforce const for never-modified variables
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        "useDefaultParameterLast": "error", // Enforce default parameters last
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        "useEnumInitializers": "error",    // Enforce enum initializers
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        "useExponentiationOperator": "error", // Enforce ** over Math.pow()
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        "useFragmentSyntax": "error",      // Enforce React fragment syntax
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        "useNumericLiterals": "error",     // Enforce numeric literals
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        "useSelfClosingElements": "error", // Enforce self-closing elements
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        "useShorthandArrayType": "error",  // Enforce T[] over Array<T>
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        "useSingleCaseStatement": "error", // Enforce single case per switch statement
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        "useSingleVarDeclarator": "error", // Enforce single variable declarator
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        "useTemplate": "error",            // Enforce template literals
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        "useWhile": "error"                // Enforce while over for loops where appropriate
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      }
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    }
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  }
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}
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```
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### Suspicious Rules
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Rules that flag potentially problematic code patterns.
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "suspicious": {
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        "recommended": true,
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        "noArrayIndexKey": "error",          // Disallow array index as key
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        "noAssignInExpressions": "error",    // Disallow assignment in expressions
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        "noAsyncPromiseExecutor": "error",   // Disallow async Promise executor
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        "noCatchAssign": "error",            // Disallow reassigning catch clause parameter
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        "noClassAssign": "error",            // Disallow reassigning class declarations
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        "noCommentText": "error",            // Disallow comments in JSX text
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        "noCompareNegZero": "error",         // Disallow comparing against -0
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        "noControlCharactersInRegex": "error", // Disallow control characters in regex
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        "noDebugger": "error",               // Disallow debugger statements
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        "noDoubleEquals": "error",           // Disallow == and !=
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        "noDuplicateCase": "error",          // Disallow duplicate case labels
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        "noDuplicateClassMembers": "error", // Disallow duplicate class members
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        "noDuplicateJsxProps": "error",     // Disallow duplicate JSX props
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        "noDuplicateObjectKeys": "error",   // Disallow duplicate object keys
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        "noDuplicateParameters": "error",   // Disallow duplicate parameters
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        "noEmptyBlockStatements": "error",  // Disallow empty block statements
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        "noExplicitAny": "error",           // Disallow explicit any type
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        "noExtraNonNullAssertion": "error", // Disallow extra non-null assertions
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        "noFallthroughSwitchClause": "error", // Disallow fallthrough switch cases
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        "noFunctionAssign": "error",        // Disallow reassigning function declarations
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        "noGlobalAssign": "error",          // Disallow assignments to native objects
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        "noImportAssign": "error",          // Disallow assigning to imports
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        "noLabelVar": "error",              // Disallow labels with variable names
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        "noMisleadingCharacterClass": "error", // Disallow misleading character class
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        "noPrototypeBuiltins": "error",     // Disallow prototype builtins
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        "noRedeclare": "error",             // Disallow variable redeclaration
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        "noRedundantUseStrict": "error",    // Disallow redundant 'use strict'
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        "noSelfCompare": "error",           // Disallow self comparison
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        "noShadowRestrictedNames": "error", // Disallow shadowing restricted names
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        "noSparseArray": "error",           // Disallow sparse arrays
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        "noUnsafeNegation": "error",        // Disallow unsafe negation
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        "useDefaultSwitchClauseLast": "error", // Enforce default clause last
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        "useGetterReturn": "error"          // Enforce return in getters
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      }
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    }
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  }
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}
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```
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## Configuration
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### Rule Configuration Levels
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Each rule can be configured with different severity levels:
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```json { .api }
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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUnusedVariables": "error",   // Error level (breaks build)
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        "noConstAssign": "warn",        // Warning level (reports but continues)
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        "noEmptyPattern": "off"         // Disabled
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      }
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    }
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  }
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}
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```
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### Rule Groups
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Enable or disable entire rule groups:
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```json { .api }
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  "linter": {
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    "enabled": true,
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    "rules": {
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      "recommended": true,    // Enable all recommended rules
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      "a11y": {
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        "recommended": true   // Enable recommended accessibility rules
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      },
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      "performance": {
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        "recommended": false, // Disable performance rules
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        "noDelete": "error"   // But enable specific rule
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      }
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    }
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  }
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}
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```
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### File Ignoring
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Configure files and patterns to ignore:
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```json { .api }
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{
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  "linter": {
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    "ignore": [
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      "dist/**",
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      "node_modules/**",
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      "*.min.js",
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      "build/**/*.js"
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    ]
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  }
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}
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```
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## Usage Examples
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### Basic Linting
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```bash
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# Check all files in src directory
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rome check src/
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# Check specific files  
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rome check src/index.js src/utils.ts
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# Check with custom file size limit
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rome check src/ --files-max-size=2097152
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```
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### Auto-fixing
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```bash
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# Apply only safe fixes
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rome check src/ --apply
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# Apply all available fixes
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rome check src/ --apply-suggested
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# Check what would be fixed without applying
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rome check src/ --verbose
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```
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### CI Integration
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```bash
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# Exit with error code if issues found
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rome check src/
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# Limit output for CI logs
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rome check src/ --max-diagnostics=50
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# JSON output for tooling integration
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rome check src/ --json
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```
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## Error Handling
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### Diagnostic Output
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Rome provides detailed diagnostic information:
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- **File location**: Exact line and column numbers
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- **Rule name**: Which rule was violated
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- **Severity level**: Error, warning, or info
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- **Fix availability**: Whether automatic fixes are available
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- **Code context**: Surrounding code for context
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### Exit Codes
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- `0`: No issues found
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- `1`: Issues found (errors or warnings based on configuration)
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- `2`: Configuration or runtime error