duplicate-code-detector

name

duplicate-code-detector

description

Use when asked about code quality or duplication. Runs jscpd analysis for quantitative metrics and TDD refactoring plans. Triggers: "duplicate code", "code quality", "find clones", "copy-paste detection". If thinking "manual review is fine" - automate instead. NOTE: For "refactor" or "technical debt" → use this skill FIRST to identify targets, THEN use incremental-refactoring to implement changes.

Duplicate Code Detector

Core principle: Systematic automated detection > manual subjective review

Workflow Clarification: Detection vs Implementation

This skill is for DETECTION (finding duplicates). For IMPLEMENTATION (refactoring them), use incremental-refactoring afterward.

Sequential workflow:

1. duplicate-code-detector → Find and prioritize duplicates
2. incremental-refactoring → Implement changes with metrics

If user says "refactor technical debt" → Start here to identify targets, then hand off to incremental-refactoring.

MANDATORY FIRST STEP

TodoWrite: Create 5 items (1 per workflow step)

1. Run jscpd analysis
2. Extract metrics
3. Dispatch subagents for top duplicates
4. Generate TDD refactoring plan
5. Present findings + verification

Prerequisites Check

which jscpd || npm install -g jscpd

If unavailable: inform user, note manual analysis limitations.

jscpd docs: https://github.com/kucherenko/jscpd

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Workflow

1. Run jscpd

jscpd --min-lines 5 --reporters json,console /path/to/code

2. Extract Metrics

• Duplication %
• Total duplicated lines
• Number of clones
• Files with most duplication

3. Dispatch Subagents

For top 3-5 duplicate groups, dispatch with:

Analyze duplicate in File A (lines X-Y) vs File B (lines M-N).
Propose extraction (function/class/module), estimate impact.

4. Generate TDD Plan

Prioritize by: lines duplicated → instances → complexity

## Priority 1: [Name] (X lines, Y instances)
1. Write test for current behavior
2. Extract [function/class/module]
3. Replace duplicates
4. Verify tests pass

5. Present Results

**Metrics:** X% duplication (Y lines, Z clone groups)
**Top 3 Priorities:** [list]
**Refactoring Plan:** [TDD steps]

Start with Priority 1?

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Verification Checkpoint

Before marking complete:

1. ✅ jscpd ran with metrics extracted
2. ✅ TDD refactoring plan includes test-first steps
3. ✅ Priorities ranked by impact (lines × instances)

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Red Flags

Thought	Reality
"Manual review is faster"	Misses 70% of duplicates
"I can spot patterns by eye"	Subjective, no metrics
"Let me read a few files first"	Run jscpd first, analyze after