check-similarity-ts

name	check-similarity-ts
description	Detect duplicate TypeScript/JavaScript code using AST-based similarity analysis. Use when working with .ts/.tsx/.js/.jsx files and looking for code duplication or refactoring opportunities.
argument-hint	[path] [--threshold 0.85] [--print]
allowed-tools	Bash(similarity-ts *) Bash(cargo run -p similarity-ts *) Read Grep Glob
paths	**/*.ts,**/*.tsx,**/*.js,**/*.jsx

TypeScript/JavaScript Code Similarity Detection

What to do

Run similarity-ts on the target project to detect duplicate functions and types, then analyze results and propose refactoring.

If similarity-ts is not installed:

cargo install similarity-ts

Step 1: Run similarity analysis

similarity-ts $ARGUMENTS

If no arguments given:

similarity-ts . --threshold 0.85 --min-tokens 25

For type-level duplicates (interfaces, type aliases):

similarity-ts . --threshold 0.85 --experimental-types

Step 2: Analyze results

High-priority

• 100% similarity: Extract shared function
• 95-100%: Parameterize the small difference
• Duplicate types/interfaces: Consolidate into a single definition and re-export

Medium-priority

• 85-95%: Extract common pattern, especially for API handlers and data processing
• Similar type literals: Shared interface with optional fields

Acceptable

• Short utility functions (< 5 lines) that naturally share structure
• Overloaded variants that differ by type parameter

Step 3: Propose refactoring

For each high-priority pair, show before/after code with concrete implementation.

Key Options

Option	Description
--threshold <0-1>	Similarity threshold (default: 0.85)
--min-tokens <n>	Skip functions with fewer AST nodes (recommended: 20-30)
--print	Show actual code snippets
--experimental-types	Enable type/interface similarity detection
--experimental-overlap	Enable partial overlap detection
--extensions <ext>	File extensions to check (comma-separated)
--filter-function <name>	Filter by function name
--filter-function-body <text>	Filter by function body content
--fail-on-duplicates	Exit code 1 if duplicates found

Common TS/JS refactoring patterns

• Data processing loops with different field names -> generic mapper
• API handlers with similar request/response logic -> shared middleware
• Validation functions -> schema-based validation
• Duplicate interfaces -> shared base interface with extensions
• Similar type literals -> extract named type