jscpd

Copy-paste detector for programming source code, supports 220+ languages. Use this skill to run jscpd and understand its output.

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>

# Second pass, noisier: copies that only differ in names and values (Type-2, "renamed").
# Review each hit before acting on it.
npx jscpd --reporters ai --ignore-identifiers --min-tokens 70 <path>

# Third pass, noisier still: copies with a few edited lines or the same function
# structure (Type-3, "similar"). Keep the settings tight.
npx jscpd --reporters ai --max-gap-lines 1 --similarity 0.85 <path>

# Where to refactor first: clone list plus a hotspot summary
npx jscpd --reporters ai --summary <path>

# Most complex files only, without clone detection
npx jscpd --complexity --reporters ai <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
src/cart/ basket.js:1-9 ~ cart.js:1-9 (renamed)
src/api/ save-account.js:1-12 ~ save-user.js:1-11 [~0.91 gap]
src/billing/ credit-note.js:1-19 ~ invoice.js:1-17 [~0.75 ast]
---
5 clones · 4.2% duplication

Each 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

A suffix tells the kind of clone; no suffix means an exact copy:

• (renamed): the two blocks differ only in identifier names, literal values or annotations (Type-2). Only appears with --ignore-identifiers, --ignore-literals or --ignore-annotations.
• [~0.91 gap]: two exact clones merged across up to --max-gap-lines unmatched lines (Type-3). The number is matched tokens over the merged span.
• [~0.75 ast]: two functions whose syntax-tree structure overlaps at least --similarity (Type-3). The number is the structural similarity, names and literal values do not count.

Options

Clone Kinds: Exact, Renamed, Similar

By default jscpd reports exact clones only: the token sequences are identical (whitespace, layout and, depending on the mode, comments do not count). Two opt-in families widen the net. Run them as separate passes after the default scan, because they find more and longer clones and change what a "clone" means.

These passes are noisy by design. An exact clone is almost always a real copy. A renamed or similar clone is a candidate: the flags deliberately ignore the very things (names, values, a statement or two) that often make two blocks different in meaning. Expect false positives from:

• boilerplate that is supposed to look alike: DTOs and models, config tables, enum-like maps, route or handler registrations, builders
• test files: describe/it blocks, fixtures and setup code repeat the same shape on purpose
• generated code, migrations, serializers, protocol bindings
• language idioms: two reduce loops or two switch statements that share structure but not logic
• small blocks: with --ignore-identifiers a 50-token block is mostly placeholders, so raise --min-tokens

Rules that keep the noise manageable:

• Run them after the exact clones are handled, one family at a time, so every hit is attributable to one flag.
• Start conservative: --ignore-identifiers alone (add --ignore-literals only when constants are the known problem), --max-gap-lines 1 or 2, --similarity 0.85 or higher, and --min-tokens 70 or more for the identifier pass. Widen only when the tight run comes back empty.
• Treat every (renamed) or [~…] line as a lead to read, not a defect to fix. Do not gate CI (--threshold, --fail-on-new-clones) on these passes unless the team has reviewed what they report on the codebase.
• Never claim "N duplicates found" from a normalized run without saying which flags produced them.

Type-2: renamed clones (--ignore-identifiers, --ignore-literals, --ignore-annotations)

Copies where someone renamed the variables or changed the constants:

npx jscpd --reporters ai --ignore-identifiers <path>                      # function a(x) {…} matches function b(y) {…}
npx jscpd --reporters ai --ignore-identifiers --ignore-literals <path>    # …and 10 matches 25, 'dev' matches 'prod'
npx jscpd --reporters ai --ignore-annotations <path>                      # @Override / @Deprecated no longer split a clone

Clones:
basket.js:1-9 ~ cart.js:1-9 (renamed)
limits-dev.js:1-13 ~ limits-prod.js:1-13 (renamed)
---

• Keywords keep their meaning (return never matches retry), so structure still has to match.
• --ignore-annotations only acts in Java, Kotlin, Scala, Groovy, Python, Dart, Swift, JavaScript and TypeScript; in Ruby, Perl, T-SQL, Razor and CSS @ means something else and is left alone.
• Positions in the report still point at the original source.
• Config keys: ignoreIdentifiers, ignoreLiterals, ignoreAnnotations.

Type-3: near-miss clones (--max-gap-lines, --similarity)

Copies with a few edited lines, or functions rewritten with the same shape:

npx jscpd --reporters ai --max-gap-lines 2 <path>      # a copy with up to 2 inserted/changed lines becomes one clone
npx jscpd --reporters ai --similarity 0.8 <path>       # JS/TS functions with ≥80% shared syntax-tree structure

Clones:
save-account.js:1-12 ~ save-user.js:1-11 [~0.91 gap]
credit-note.js:1-19 ~ invoice.js:1-17 [~0.75 ast]
---

• --max-gap-lines N only joins clones the exact run already found, so it removes fragmentation rather than inventing matches; it works in every language. A merge is refused when the gap holds more tokens than the halves share (similarity would drop under 0.5).
• --similarity RATIO compares whole functions by the bag of 4-grams over their syntax-tree node types, so a renamed copy scores 1.0, one inserted line about 0.9, two added statements plus renames about 0.75. Today it applies to JavaScript, TypeScript, JSX and TSX only; other formats are a silent no-op. Start at 0.85 for near-identical structure and lower to 0.7 only when looking for leads; below 0.8 a large share of pairs merely share an idiom, so read both functions before believing the score.
• similar takes precedence over renamed when both apply (a merged clone is no longer identical even after normalization).
• Config keys: maxGapLines, similarity.

Where the kind shows up

• console: Clone found (javascript, renamed), Clone found (javascript, similar (gap) ~0.91), Clone found (javascript, similar (ast) ~0.75).
• json: "kind": "exact" | "renamed" | "similar", plus "similarity" and "method": "gap" | "ast" for similar clones.
• sarif: rules jscpd/duplicate-code, jscpd/renamed-code, jscpd/similar-code; Code Climate uses the same three check_name values.
• A default run reports only exact clones and its output is unchanged by these features.
• Normalized runs produce different clone fingerprints than exact runs: keep a separate --baseline file per configuration.

Codebase Summary (--summary)

--summary appends a refactoring-hotspot overview to the run output — use it to decide where to refactor first before diving into individual clones:

# Compact clone list + compact summary, optimized for agents
npx jscpd --reporters ai --summary --no-tips <path>

# Rank by complexity instead of tokens, top 5 lists
npx jscpd --reporters ai --summary --summary-by complexity --summary-top 5 <path>

With the ai reporter the summary is one line per entry:

Summary by tokens (321 files, 129 folders):
files (tokens/lines/size/cx/dup%):
src/files.ts 2052/363/11662/80/0.0%
long-line/theme-branded.js 498/15/2.5K/10/93.3%
...
folders (files/tokens/lines/size):
src/core 8/5264/843/27136
...

How to read it:

• Top files are ranked by the --summary-by metric, but every row carries all metrics — tokens/lines/size/cx/dup%.
• cx is a language-agnostic cyclomatic-complexity estimate from the token stream (1 + decision-point tokens like if/while/&&); treat it as a ranking signal, not an exact metric.
• dup% is the share of the file's lines covered by detected clones — a large file with high dup% is the best refactoring target. It reflects the clones of the current run, so with the Type-2/Type-3 flags it rises accordingly.
• Folders aggregate files into their direct parent directory (no cumulative ancestor totals).
• In console reporters the summary renders as aligned tables; in the json report it appears as an additive summary key (absent when the flag is off).

Config file equivalents: "summary": true, "summaryTop": 10, "summaryBy": "tokens".

Cross-Format Clone Detection

By default each format is compared only against itself. --cross-formats defines groups of related formats that share one comparison pool, so a block duplicated between a .js and a .ts file is reported as a clone:

# One group: compare JavaScript and TypeScript files together
npx jscpd --reporters ai --cross-formats "javascript,typescript" <path>

# Preset covering javascript, jsx, typescript, tsx
npx jscpd --reporters ai --cross-formats "js-ts" <path>

# Multiple groups are separated by ";"
npx jscpd --reporters ai --cross-formats "javascript,typescript;css,scss" <path>

Notes:

• When a group mixes TypeScript with JavaScript, TS files are compared with erasable type syntax stripped, so function f(a: number): void matches function f(a). Reported positions still reference the original source.
• Groups need at least two formats; groups sharing a format are merged into one pool.
• In per-format statistics, a cross-format clone is attributed to one member format of the group.
• In config files the key is crossFormats (or cross-formats) and accepts a string ("javascript,typescript;css,scss"), an array of strings (["javascript,typescript", "css,scss"]), or an array of arrays ([["javascript","typescript"],["css","scss"]]).

Configuration File

Create a .jscpd.json in your project root:

{
  "threshold": 0,
  "reporters": ["ai"],
  "ignore": ["**/node_modules/**", "**/dist/**", "**/*.min.*"],
  "format": ["typescript", "javascript"],
  "minLines": 5,
  "minTokens": 50,
  "ignoreIdentifiers": false,
  "ignoreLiterals": false,
  "maxGapLines": 0,
  "similarity": 1,
  "summary": false,
  "output": "./reports/jscpd"
}

Refactoring Duplicated Code

Once you've detected clones, use the dry-refactoring skill for a guided workflow to eliminate them, with a strategy per clone kind:

→ dry-refactoring — step-by-step refactoring strategies and workflow for removing duplication. Install with:

npx skills add https://github.com/kucherenko/jscpd --skill dry-refactoring

Improving Overall Codebase Health

For a broader "clean up this codebase" pass — duplication, then dead code, then the largest/most complex files, prioritized from --health/--dashboard and re-measured at the end — use codebase-refactoring:

npx skills add https://github.com/kucherenko/jscpd --skill codebase-refactoring