mizchi 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