What jscpd finds

jscpd finds duplicated code in a repository and reports it in the terminal, in CI, in an editor or to a coding agent. It is for teams that want a duplication number to gate a build on, and the copies behind it.

What a default run reports

Without options, jscpd reports exact copies: every block of at least 50 tokens and 5 lines that appears more than once (--min-tokens, --min-lines). It reads 224 formats and tokenizes each with that language's own comment and string rules, JavaScript and TypeScript through the oxc parser, so a clone is a repeated run of language tokens. A rolling Rabin-Karp hash over those tokens finds the repeats. Inside a git repository, files that .gitignore lists are skipped. How detection works follows a file through the pipeline; Supported formats lists the formats.

jscpd .

Clone found (javascript)
 - generated/checkout.js [2:1 - 25:2] (24 lines, 179 tokens)
   src/checkout.js [1:1 - 24:2]
Clone found (javascript)
 - generated/checkout.js [2:1 - 25:2] (24 lines, 179 tokens)
   vendor/checkout.js [2:1 - 25:2]
Found 2 clones.

After the clones, the terminal prints one row per format and a total; for this run it is one row:

The run is fixtures/ignore-demo/glob from the jscpd repository: one file copied into vendor/ and generated/. Each clone names both places with line and column; the percentage is the duplication of the scan.

What it finds when you ask

• Renamed copies, the same code under other variable, function or literal names: --ignore-identifiers, --ignore-literals and --ignore-annotations.
• Near-miss copies with a few inserted or edited lines: --max-gap-lines; and JavaScript or TypeScript functions with the same structure: --similarity.
• Semantic clones, functions that do the same job with different code, in one language or across languages: --semantic, experimental, with a code embedding model.

Types of code clones says where the lines between these sit and which flag turns each on.

What it does not do

jscpd does not rewrite code: it reports, and the refactoring is yours or your agent's. A scan reads the paths you pass and makes no network call; there is no hosted service and no index of other repositories to compare against. The semantic mode is the exception: it downloads a 548 MB model once, or talks to an embeddings API you name. A default run reports exact clones only; the other kinds need their flags.

Other questions jscpd answers

--dashboard and --health put duplication, dead code and complexity on one screen with a score (dashboard, health score). --dead-code finds unused files, exports, symbols and imports in JavaScript, TypeScript and Python (dead code). --history plots duplication over git history (history), and --compare measures a port or two implementations of one app (compare). --mcp serves the detector to coding agents (agents) and --lsp to editors (editors).

Where to go next