jscpd
Copy-paste detector for programming source code, supports 150+ languages. Use this skill to find duplicated code and refactor it away.
Quick Start
# Run with ai reporter (compact output optimized for agents)
npx jscpd --reporters ai <path>
# With ignore patterns
npx jscpd --reporters ai --ignore "**/node_modules/**,**/dist/**" <path>
# Scope to specific formats
npx jscpd --reporters ai --format "javascript,typescript" <path>AI Reporter Output Format
The ai reporter produces compact, token-efficient output designed for agent consumption:
Clones:
src/ foo.ts:10-25 ~ bar.ts:42-57
src/utils/helpers.ts:100-120 ~ src/utils/other.ts:5-25
---
3 clones · 4.2% duplicationEach line represents one clone pair:
- Same file:
path/file.ts 10-25 ~ 45-60(shared path shown once) - Same directory:
shared/prefix/ file-a.ts:10-25 ~ file-b.ts:42-57(common prefix factored out) - Different paths:
path/a.ts:10-25 ~ path/b.ts:42-57
Common Options
| Option | Description |
|---|---|
--reporters ai | Use the AI-optimized reporter (required for this skill) |
--min-tokens N | Minimum tokens to consider a duplication (default: 50) |
--min-lines N | Minimum lines to consider a duplication (default: 5) |
--threshold N | Exit with error if duplication % exceeds N |
--ignore "glob" | Ignore patterns (comma-separated) |
--format "list" | Limit to specific languages (e.g. typescript,javascript) |
--pattern "glob" | Glob pattern to select files |
--gitignore | Respect.gitignore |
--silent | Suppress output (useful with --output only) |
Workflow
- Run jscpd with
--reporters aion the target path - Parse each clone line to identify the two duplicated locations (file + line range)
- Read both code fragments from the source files
- Understand what the duplicated code does
- Design a refactoring: extract a shared function, class, module, or constant
- Apply the refactoring — update both locations and all other usages
- Re-run jscpd to confirm the clone is eliminated
- Repeat for remaining clones, highest-impact first
Refactoring Strategies
Extract function — when the duplicate is a block of logic:
// Before: same block in two places
// After: shared function called from both placesExtract module/utility — when the duplicate spans multiple files in different domains:
// Move shared logic to a shared utility file and import itExtract constant or config — when the duplicate is repeated data or configuration.
Template/base class — when the duplicate is structural (e.g., repeated class shape).
Always ensure:
- All call sites are updated, not just the two reported by jscpd
- Tests still pass after refactoring
- The extracted abstraction has a clear, descriptive name
Tips
- Start with clones that have the highest line count — they have the most impact
- A clone between test files may indicate a missing test helper
- Clones across unrelated modules may signal a missing shared utility
- Use
--min-lines 10to filter noise and focus on meaningful duplications