# Refactoring Catalog > Identifies code smells and applies refactoring techniques from Martin Fowler's catalog. Use when improving code structure, reducing complexity, or eliminating smells without changing behavior. - Skill: `majiayu000/refactoring-catalog` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds add majiayu000/refactoring-catalog` - Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/refactoring-catalog/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: majiayu000 (https://skillmd.com/u/majiayu000) - Updated: 2026-09-09 - Page: https://skillmd.com/skills/majiayu000/refactoring-catalog --- # Refactoring Catalog Systematic techniques for improving code structure without changing its observable behavior. Based on Martin Fowler's Refactoring Catalog. --- ## Composing Methods ### Extract Function **Smell**: Long method, code fragment that can be grouped **Action**: Turn the fragment into a method with a name explaining its purpose **Cookbook**: [extract-function.md](./cookbook/extract-function.md) ### Inline Function **Smell**: Function body is as clear as its name **Action**: Replace calls with the function body and remove the function **Cookbook**: [inline-function.md](./cookbook/inline-function.md) ### Extract Variable **Smell**: Complex expression that's hard to understand **Action**: Place the result in a temporary variable with a meaningful name **Cookbook**: [extract-variable.md](./cookbook/extract-variable.md) ### Inline Variable **Smell**: Variable name doesn't communicate more than the expression **Action**: Replace references with the expression itself **Cookbook**: [inline-variable.md](./cookbook/inline-variable.md) ### Replace Temp with Query **Smell**: Temporary variable holding the result of an expression **Action**: Extract the expression into a method and replace temp with query **Cookbook**: [replace-temp-with-query.md](./cookbook/replace-temp-with-query.md) ### Split Variable **Smell**: Variable assigned more than once (not a loop variable) **Action**: Make a separate variable for each assignment **Cookbook**: [split-variable.md](./cookbook/split-variable.md) --- ## Moving Features ### Move Function **Smell**: Function references elements from another context more than its own **Action**: Move it to the context it references most **Cookbook**: [move-function.md](./cookbook/move-function.md) ### Move Field **Smell**: Field used more by another class than its own **Action**: Move it to the class that uses it most **Cookbook**: [move-field.md](./cookbook/move-field.md) ### Move Statements into Function **Smell**: Same code executed around a function call **Action**: Move those statements into the function **Cookbook**: [move-statements-into-function.md](./cookbook/move-statements-into-function.md) ### Move Statements to Callers **Smell**: Function with behavior that some callers don't need **Action**: Move the varying behavior to the callers **Cookbook**: [move-statements-to-callers.md](./cookbook/move-statements-to-callers.md) ### Replace Inline Code with Function Call **Smell**: Inline code doing the same thing as an existing function **Action**: Replace with a call to the function **Cookbook**: [replace-inline-code-with-function-call.md](./cookbook/replace-inline-code-with-function-call.md) ### Slide Statements **Smell**: Related code fragments not next to each other **Action**: Slide statements to be adjacent **Cookbook**: [slide-statements.md](./cookbook/slide-statements.md) ### Split Loop **Smell**: Loop doing multiple things **Action**: Split into separate loops for each task **Cookbook**: [split-loop.md](./cookbook/split-loop.md) ### Split Phase **Smell**: Code dealing with two different things **Action**: Split into separate phases with intermediate data structure **Cookbook**: [split-phase.md](./cookbook/split-phase.md) --- ## Organizing Data ### Replace Primitive with Object **Smell**: Primitive used to represent a domain concept **Action**: Create a class with the primitive as its data **Cookbook**: [replace-primitive-with-object.md](./cookbook/replace-primitive-with-object.md) ### Change Value to Reference **Smell**: Multiple copies of the same conceptual data **Action**: Replace copies with a single reference object **Cookbook**: [change-value-to-reference.md](./cookbook/change-value-to-reference.md) ### Change Reference to Value **Smell**: Reference object that's hard to work with **Action**: Replace with a value object **Cookbook**: [change-reference-to-value.md](./cookbook/change-reference-to-value.md) ### Replace Magic Literal **Smell**: Literal with special meaning that's not obvious **Action**: Replace with a named constant **Cookbook**: [replace-magic-literal.md](./cookbook/replace-magic-literal.md) ### Rename Variable **Smell**: Variable name doesn't clearly communicate its purpose **Action**: Change the name to something more communicative **Cookbook**: [rename-variable.md](./cookbook/rename-variable.md) ### Rename Field **Smell**: Field name doesn't clearly communicate its purpose **Action**: Change the name to something more communicative **Cookbook**: [rename-field.md](./cookbook/rename-field.md) ### Change Function Declaration **Smell**: Function name/parameters don't clearly communicate purpose **Action**: Rename function and/or add/remove parameters **Cookbook**: [change-function-declaration.md](./cookbook/change-function-declaration.md) ### Encapsulate Variable **Smell**: Public field accessed directly **Action**: Create getter/setter functions **Cookbook**: [encapsulate-variable.md](./cookbook/encapsulate-variable.md) ### Encapsulate Collection **Smell**: Collection returned directly, allowing modification **Action**: Return a copy or read-only view **Cookbook**: [encapsulate-collection.md](./cookbook/encapsulate-collection.md) ### Encapsulate Record **Smell**: Record/hash used for mutable data **Action**: Create a class with accessors **Cookbook**: [encapsulate-record.md](./cookbook/encapsulate-record.md) --- ## Simplifying Conditional Logic ### Decompose Conditional **Smell**: Complex conditional (if-else) with complicated conditions **Action**: Extract condition and each branch into separate functions **Cookbook**: [decompose-conditional.md](./cookbook/decompose-conditional.md) ### Consolidate Conditional Expression **Smell**: Multiple conditionals with the same result **Action**: Combine them into a single conditional **Cookbook**: [consolidate-conditional-expression.md](./cookbook/consolidate-conditional-expression.md) ### Replace Nested Conditional with Guard Clauses **Smell**: Deeply nested if-else structure **Action**: Use guard clauses for special cases, return early **Cookbook**: [replace-nested-conditional-with-guard-clauses.md](./cookbook/replace-nested-conditional-with-guard-clauses.md) ### Replace Conditional with Polymorphism **Smell**: Switch/if-else selecting behavior based on type **Action**: Create subclasses and override methods **Cookbook**: [replace-conditional-with-polymorphism.md](./cookbook/replace-conditional-with-polymorphism.md) ### Introduce Special Case **Smell**: Many places check for a special value (often null) **Action**: Create a special case object encapsulating the behavior **Cookbook**: [introduce-special-case.md](./cookbook/introduce-special-case.md) ### Introduce Assertion **Smell**: Code assumes something about program state **Action**: Make the assumption explicit with an assertion **Cookbook**: [introduce-assertion.md](./cookbook/introduce-assertion.md) ### Replace Control Flag with Break **Smell**: Boolean variable controlling loop exit **Action**: Use break, continue, or return instead **Cookbook**: [replace-control-flag-with-break.md](./cookbook/replace-control-flag-with-break.md) ### Replace Exception with Precheck **Smell**: Exception used for expected condition **Action**: Check the condition first **Cookbook**: [replace-exception-with-precheck.md](./cookbook/replace-exception-with-precheck.md) --- ## Refactoring APIs ### Separate Query from Modifier **Smell**: Function that returns a value and has side effects **Action**: Create two functions: one for query, one for modifier **Cookbook**: [separate-query-from-modifier.md](./cookbook/separate-query-from-modifier.md) ### Parameterize Function **Smell**: Multiple functions doing similar things with literals **Action**: Combine into one function with a parameter **Cookbook**: [parameterize-function.md](./cookbook/parameterize-function.md) ### Remove Flag Argument **Smell**: Boolean argument that changes function behavior **Action**: Create separate functions for each case **Cookbook**: [remove-flag-argument.md](./cookbook/remove-flag-argument.md) ### Preserve Whole Object **Smell**: Passing several values from one object to a function **Action**: Pass the whole object instead **Cookbook**: [preserve-whole-object.md](./cookbook/preserve-whole-object.md) ### Replace Parameter with Query **Smell**: Parameter that can be derived from another parameter **Action**: Remove the parameter and let the function calculate it **Cookbook**: [replace-parameter-with-query.md](./cookbook/replace-parameter-with-query.md) ### Replace Query with Parameter **Smell**: Function references a global or internal value directly **Action**: Pass the value as a parameter **Cookbook**: [replace-query-with-parameter.md](./cookbook/replace-query-with-parameter.md) ### Remove Setting Method **Smell**: Field should be set at construction time only **Action**: Remove the setter and set in constructor **Cookbook**: [remove-setting-method.md](./cookbook/remove-setting-method.md) ### Replace Constructor with Factory Function **Smell**: Constructor has limitations (name, return type) **Action**: Replace with a factory function **Cookbook**: [replace-constructor-with-factory-function.md](./cookbook/replace-constructor-with-factory-function.md) ### Replace Function with Command **Smell**: Function needs to be undone, queued, or has complex state **Action**: Encapsulate the function in a command object **Cookbook**: [replace-function-with-command.md](./cookbook/replace-function-with-command.md) ### Replace Command with Function **Smell**: Command object is too simple **Action**: Replace with a plain function **Cookbook**: [replace-command-with-function.md](./cookbook/replace-command-with-function.md) ### Return Modified Value **Smell**: Function updates data but doesn't return it **Action**: Return the modified value **Cookbook**: [return-modified-value.md](./cookbook/return-modified-value.md) ### Replace Error Code with Exception **Smell**: Function returns error code to indicate failure **Action**: Throw an exception instead **Cookbook**: [replace-error-code-with-exception.md](./cookbook/replace-error-code-with-exception.md) ### Introduce Parameter Object **Smell**: Group of parameters that travel together **Action**: Replace them with an object **Cookbook**: [introduce-parameter-object.md](./cookbook/introduce-parameter-object.md) --- ## Dealing with Inheritance ### Pull Up Method **Smell**: Identical methods in sibling classes **Action**: Move to the superclass **Cookbook**: [pull-up-method.md](./cookbook/pull-up-method.md) ### Pull Up Field **Smell**: Identical field in sibling classes **Action**: Move to the superclass **Cookbook**: [pull-up-field.md](./cookbook/pull-up-field.md) ### Pull Up Constructor Body **Smell**: Constructors in subclasses have common code **Action**: Move common code to superclass constructor **Cookbook**: [pull-up-constructor-body.md](./cookbook/pull-up-constructor-body.md) ### Push Down Method **Smell**: Method only relevant to one subclass **Action**: Move it to that subclass **Cookbook**: [push-down-method.md](./cookbook/push-down-method.md) ### Push Down Field **Smell**: Field only used by one subclass **Action**: Move it to that subclass **Cookbook**: [push-down-field.md](./cookbook/push-down-field.md) ### Replace Type Code with Subclasses **Smell**: Field/parameter that indicates type with conditional behavior **Action**: Replace with subclasses **Cookbook**: [replace-type-code-with-subclasses.md](./cookbook/replace-type-code-with-subclasses.md) ### Remove Subclass **Smell**: Subclass does too little to justify its existence **Action**: Replace with a field in the superclass **Cookbook**: [remove-subclass.md](./cookbook/remove-subclass.md) ### Extract Superclass **Smell**: Multiple classes with similar features **Action**: Create a superclass with the common elements **Cookbook**: [extract-superclass.md](./cookbook/extract-superclass.md) ### Collapse Hierarchy **Smell**: Subclass is no different from its superclass **Action**: Merge them together **Cookbook**: [collapse-hierarchy.md](./cookbook/collapse-hierarchy.md) ### Replace Subclass with Delegate **Smell**: Inheritance used for a single axis of variation **Action**: Replace with a delegate field **Cookbook**: [replace-subclass-with-delegate.md](./cookbook/replace-subclass-with-delegate.md) ### Replace Superclass with Delegate **Smell**: Subclass doesn't want all of superclass behavior **Action**: Replace inheritance with delegation **Cookbook**: [replace-superclass-with-delegate.md](./cookbook/replace-superclass-with-delegate.md) ### Extract Class **Smell**: Class doing too much, has too many responsibilities **Action**: Split into two classes **Cookbook**: [extract-class.md](./cookbook/extract-class.md) --- ## Other Refactorings ### Inline Class **Smell**: Class does too little to justify its existence **Action**: Move all features to another class **Cookbook**: [inline-class.md](./cookbook/inline-class.md) ### Hide Delegate **Smell**: Client calls through one object to reach another **Action**: Create a delegating method **Cookbook**: [hide-delegate.md](./cookbook/hide-delegate.md) ### Remove Middle Man **Smell**: Class has too many simple delegating methods **Action**: Let clients call the delegate directly **Cookbook**: [remove-middle-man.md](./cookbook/remove-middle-man.md) ### Substitute Algorithm **Smell**: Algorithm can be replaced with a clearer one **Action**: Replace the body with the new algorithm **Cookbook**: [substitute-algorithm.md](./cookbook/substitute-algorithm.md) ### Combine Functions into Class **Smell**: Group of functions operating on the same data **Action**: Form a class with the data and functions **Cookbook**: [combine-functions-into-class.md](./cookbook/combine-functions-into-class.md) ### Combine Functions into Transform **Smell**: Reading data, computing derived values, passing around **Action**: Create a transform function that enriches the data **Cookbook**: [combine-functions-into-transform.md](./cookbook/combine-functions-into-transform.md) ### Replace Loop with Pipeline **Smell**: Loop processing a collection **Action**: Replace with pipeline operations (map, filter, reduce) **Cookbook**: [replace-loop-with-pipeline.md](./cookbook/replace-loop-with-pipeline.md) ### Remove Dead Code **Smell**: Code that's never executed **Action**: Delete it **Cookbook**: [remove-dead-code.md](./cookbook/remove-dead-code.md) ### Replace Derived Variable with Query **Smell**: Variable calculated from other values and stored **Action**: Calculate it on demand **Cookbook**: [replace-derived-variable-with-query.md](./cookbook/replace-derived-variable-with-query.md) --- ## Quick Reference Table | Category | Refactoring | Primary Smell | | ------------------------ | --------------------------------------------- | ---------------------------- | | Composing Methods | Extract Function | Long method, code fragment | | Composing Methods | Inline Function | Trivial function body | | Composing Methods | Extract Variable | Complex expression | | Composing Methods | Inline Variable | Unnecessary variable | | Composing Methods | Replace Temp with Query | Temp holding expression | | Composing Methods | Split Variable | Multiple assignments | | Moving Features | Move Function | Feature envy | | Moving Features | Move Field | Field envy | | Moving Features | Move Statements into Function | Repeated code around call | | Moving Features | Move Statements to Callers | Varying behavior in function | | Moving Features | Replace Inline Code with Function Call | Duplicate logic | | Moving Features | Slide Statements | Separated related code | | Moving Features | Split Loop | Loop doing multiple things | | Moving Features | Split Phase | Mixed concerns | | Organizing Data | Replace Primitive with Object | Primitive obsession | | Organizing Data | Change Value to Reference | Inconsistent data | | Organizing Data | Change Reference to Value | Complex reference handling | | Organizing Data | Replace Magic Literal | Mysterious numbers | | Organizing Data | Rename Variable | Unclear naming | | Organizing Data | Rename Field | Unclear naming | | Organizing Data | Change Function Declaration | Bad signature | | Organizing Data | Encapsulate Variable | Public field | | Organizing Data | Encapsulate Collection | Exposed collection | | Organizing Data | Encapsulate Record | Naked record | | Simplifying Conditionals | Decompose Conditional | Complex conditional | | Simplifying Conditionals | Consolidate Conditional Expression | Duplicate conditions | | Simplifying Conditionals | Replace Nested Conditional with Guard Clauses | Deep nesting | | Simplifying Conditionals | Replace Conditional with Polymorphism | Type-based switching | | Simplifying Conditionals | Introduce Special Case | Null checks | | Simplifying Conditionals | Introduce Assertion | Hidden assumptions | | Simplifying Conditionals | Replace Control Flag with Break | Control flag | | Simplifying Conditionals | Replace Exception with Precheck | Exception for expected case | | Refactoring APIs | Separate Query from Modifier | Mixed query/command | | Refactoring APIs | Parameterize Function | Similar functions | | Refactoring APIs | Remove Flag Argument | Boolean argument | | Refactoring APIs | Preserve Whole Object | Data clump | | Refactoring APIs | Replace Parameter with Query | Derivable parameter | | Refactoring APIs | Replace Query with Parameter | Hidden dependency | | Refactoring APIs | Remove Setting Method | Mutable after construction | | Refactoring APIs | Replace Constructor with Factory Function | Constructor limitations | | Refactoring APIs | Replace Function with Command | Complex function | | Refactoring APIs | Replace Command with Function | Over-engineered command | | Refactoring APIs | Return Modified Value | Side effect only | | Refactoring APIs | Replace Error Code with Exception | Error code | | Refactoring APIs | Introduce Parameter Object | Data clump | | Dealing with Inheritance | Pull Up Method | Duplicate methods | | Dealing with Inheritance | Pull Up Field | Duplicate fields | | Dealing with Inheritance | Pull Up Constructor Body | Duplicate constructor code | | Dealing with Inheritance | Push Down Method | Unused in superclass | | Dealing with Inheritance | Push Down Field | Unused field | | Dealing with Inheritance | Replace Type Code with Subclasses | Type code | | Dealing with Inheritance | Remove Subclass | Trivial subclass | | Dealing with Inheritance | Extract Superclass | Duplicate features | | Dealing with Inheritance | Collapse Hierarchy | Identical class levels | | Dealing with Inheritance | Replace Subclass with Delegate | Single variation axis | | Dealing with Inheritance | Replace Superclass with Delegate | Partial inheritance | | Dealing with Inheritance | Extract Class | Large class | | Other | Inline Class | Lazy class | | Other | Hide Delegate | Law of Demeter violation | | Other | Remove Middle Man | Message chain wrapper | | Other | Substitute Algorithm | Inferior algorithm | | Other | Combine Functions into Class | Feature envy group | | Other | Combine Functions into Transform | Derived data passing | | Other | Replace Loop with Pipeline | Loop collection processing | | Other | Remove Dead Code | Dead code | | Other | Replace Derived Variable with Query | Stored calculation | --- ## Cookbook Index All 66 refactoring techniques with detailed mechanics: **Composing Methods (6)**: [Extract Function](./cookbook/extract-function.md) · [Inline Function](./cookbook/inline-function.md) · [Extract Variable](./cookbook/extract-variable.md) · [Inline Variable](./cookbook/inline-variable.md) · [Replace Temp with Query](./cookbook/replace-temp-with-query.md) · [Split Variable](./cookbook/split-variable.md) **Moving Features (8)**: [Move Function](./cookbook/move-function.md) · [Move Field](./cookbook/move-field.md) · [Move Statements into Function](./cookbook/move-statements-into-function.md) · [Move Statements to Callers](./cookbook/move-statements-to-callers.md) · [Replace Inline Code with Function Call](./cookbook/replace-inline-code-with-function-call.md) · [Slide Statements](./cookbook/slide-statements.md) · [Split Loop](./cookbook/split-loop.md) · [Split Phase](./cookbook/split-phase.md) **Organizing Data (10)**: [Replace Primitive with Object](./cookbook/replace-primitive-with-object.md) · [Change Value to Reference](./cookbook/change-value-to-reference.md) · [Change Reference to Value](./cookbook/change-reference-to-value.md) · [Replace Magic Literal](./cookbook/replace-magic-literal.md) · [Rename Variable](./cookbook/rename-variable.md) · [Rename Field](./cookbook/rename-field.md) · [Change Function Declaration](./cookbook/change-function-declaration.md) · [Encapsulate Variable](./cookbook/encapsulate-variable.md) · [Encapsulate Collection](./cookbook/encapsulate-collection.md) · [Encapsulate Record](./cookbook/encapsulate-record.md) **Simplifying Conditional Logic (8)**: [Decompose Conditional](./cookbook/decompose-conditional.md) · [Consolidate Conditional Expression](./cookbook/consolidate-conditional-expression.md) · [Replace Nested Conditional with Guard Clauses](./cookbook/replace-nested-conditional-with-guard-clauses.md) · [Replace Conditional with Polymorphism](./cookbook/replace-conditional-with-polymorphism.md) · [Introduce Special Case](./cookbook/introduce-special-case.md) · [Introduce Assertion](./cookbook/introduce-assertion.md) · [Replace Control Flag with Break](./cookbook/replace-control-flag-with-break.md) · [Replace Exception with Precheck](./cookbook/replace-exception-with-precheck.md) **Refactoring APIs (13)**: [Separate Query from Modifier](./cookbook/separate-query-from-modifier.md) · [Parameterize Function](./cookbook/parameterize-function.md) · [Remove Flag Argument](./cookbook/remove-flag-argument.md) · [Preserve Whole Object](./cookbook/preserve-whole-object.md) · [Replace Parameter with Query](./cookbook/replace-parameter-with-query.md) · [Replace Query with Parameter](./cookbook/replace-query-with-parameter.md) · [Remove Setting Method](./cookbook/remove-setting-method.md) · [Replace Constructor with Factory Function](./cookbook/replace-constructor-with-factory-function.md) · [Replace Function with Command](./cookbook/replace-function-with-command.md) · [Replace Command with Function](./cookbook/replace-command-with-function.md) · [Return Modified Value](./cookbook/return-modified-value.md) · [Replace Error Code with Exception](./cookbook/replace-error-code-with-exception.md) · [Introduce Parameter Object](./cookbook/introduce-parameter-object.md) **Dealing with Inheritance (12)**: [Pull Up Method](./cookbook/pull-up-method.md) · [Pull Up Field](./cookbook/pull-up-field.md) · [Pull Up Constructor Body](./cookbook/pull-up-constructor-body.md) · [Push Down Method](./cookbook/push-down-method.md) · [Push Down Field](./cookbook/push-down-field.md) · [Replace Type Code with Subclasses](./cookbook/replace-type-code-with-subclasses.md) · [Remove Subclass](./cookbook/remove-subclass.md) · [Extract Superclass](./cookbook/extract-superclass.md) · [Collapse Hierarchy](./cookbook/collapse-hierarchy.md) · [Replace Subclass with Delegate](./cookbook/replace-subclass-with-delegate.md) · [Replace Superclass with Delegate](./cookbook/replace-superclass-with-delegate.md) · [Extract Class](./cookbook/extract-class.md) **Other Refactorings (9)**: [Inline Class](./cookbook/inline-class.md) · [Hide Delegate](./cookbook/hide-delegate.md) · [Remove Middle Man](./cookbook/remove-middle-man.md) · [Substitute Algorithm](./cookbook/substitute-algorithm.md) · [Combine Functions into Class](./cookbook/combine-functions-into-class.md) · [Combine Functions into Transform](./cookbook/combine-functions-into-transform.md) · [Replace Loop with Pipeline](./cookbook/replace-loop-with-pipeline.md) · [Remove Dead Code](./cookbook/remove-dead-code.md) · [Replace Derived Variable with Query](./cookbook/replace-derived-variable-with-query.md)