dslop

Find duplicate functions, types, and code in your codebase using AST analysis.

``bash
npx dslop
`

What it finds

Real duplicates, not noise. dslop uses AST parsing to find semantically identical code - functions with the same logic but different variable names, types defined in multiple places, copy-pasted utilities.

`bash
$ dslop --all --json

{
"summary": { "duplicateGroups": 101 },
"duplicates": [
{
"type": "function",
"name": "loadEnv",
"occurrences": 6,
"locations": [
{ "name": "loadEnv", "file": "scripts/migrate.ts", "line": 8 },
{ "name": "loadEnv", "file": "scripts/seed.ts", "line": 12 },
{ "name": "loadEnv", "file": "lib/db/migrate.ts", "line": 7 },
{ "name": "loadEnv", "file": "lib/db/check-state.ts", "line": 9 }
]
},
{
"type": "interface",
"name": "Params",
"occurrences": 6,
"locations": [
{ "name": "Params", "file": "app/api/games/[id]/route.ts", "line": 5 },
{ "name": "Params", "file": "app/api/games/[id]/state/route.ts", "line": 8 },
{ "name": "Params", "file": "app/api/games/[id]/details/route.ts", "line": 12 }
]
},
{
"type": "type",
"name": "LeaderboardPlayer",
"occurrences": 3,
"locations": [
{ "name": "LeaderboardPlayer", "file": "packages/types/leaderboard.ts", "line": 45, "exported": true },
{ "name": "LeaderboardPlayer", "file": "apps/mobile/types.ts", "line": 52, "exported": false },
{ "name": "LeaderboardPlayer", "file": "apps/web/lib/types.ts", "line": 30, "exported": false }
]
}
]
}
`

$3

Find types and functions duplicated across packages:

`bash
$ dslop --all --cross-package

Found 48 duplicate functions/types

Types duplicated between packages/types and apps/

PrizeDistribution packages/types/game.ts ↔ packages/db/schema/game.ts
DevicePlatform apps/web/lib/notifications.ts ↔ apps/listener/lib/notifications.ts
TeamColors apps/mobile/store/types.ts ↔ apps/web/lib/types/colors.ts

Functions copy-pasted between apps/

subscribeToChannel apps/web/lib/subscriptions.ts ↔ apps/listener/lib/subscriptions.ts
getTeamLogoUrl packages/shared/logos.ts → also in apps/web (3 copies)
normalizeError apps/web/sentry.config.ts ↔ apps/listener/lib/sentry.ts
`

$3

By default, dslop checks your branch changes against the existing codebase:

`bash
$ dslop

Scanning...
Mode: checking changed lines in 3 files

Found 2 duplicate functions/types

You're adding code that already exists elsewhere:


getUserDisplayName your change: app/profile/page.tsx:19
exists in: components/sidebar.tsx:50
`

Install

`bash
npm i -g dslop
`

Usage

`bash
dslop # check PR changes (or full scan if none)
dslop --all # full codebase scan
dslop --all --json # JSON output for tooling
dslop --cross-package # only cross-package dupes (monorepos)
dslop ./apps/web # scan specific directory
`

Options

| Flag | Description |
|------|-------------|
| -a, --all | force full codebase scan |
| -c, --changes | force changes-only mode (exit if no changes) |
| -m, --min-lines | min lines per block (default: 4) |
| -s, --similarity | similarity threshold 0-100 (default: 70) |
| -e, --extensions | file extensions (default: ts,tsx,js,jsx) |
| --cross-package | only show dupes across packages/apps |
| --json | JSON output |

How it works

dslop parses TypeScript/JavaScript with Babel and extracts functions, classes, types, and interfaces. It normalizes the AST by replacing all identifiers with generic placeholders ($0, $1, etc.), preserving only the code structure.

This catches:
- Functions with identical logic but different variable names
- Types/interfaces defined in multiple places
- Copy-pasted utilities across packages

Example: these two functions are detected as duplicates:

`ts
// apps/web/utils.ts
function getUserInitials(user: User): string {
const first = user.firstName?.[0] ?? '';
const last = user.lastName?.[0] ?? '';
return (first + last).toUpperCase();
}

// apps/admin/helpers.ts
function getInitials(person: Person): string {
const f = person.firstName?.[0] ?? '';
const l = person.lastName?.[0] ?? '';
return (f + l).toUpperCase();
}
`

$3

- Same-file duplicates (patterns within a single file)
- Tiny functions (configurable via --min-lines)
- Common patterns from UI libraries (shadcn components, etc.)

Performance

dslop uses oxc (Rust-based parser) + caching for speed:

`bash


First run (builds cache)

$ dslop --all
Scanned 1410 files (373K lines) in 950ms

Cached runs

$ dslop --all
Scanned 1410 files (373K lines) in 170ms
`

Cache is stored in .dslop-cache. Add it to .gitignore. Use --no-cache` to bypass.

Limitations

- TypeScript/JavaScript only: AST parsing uses Babel with TS/JSX plugins
- No cross-language: Won't detect duplication across languages

License

MIT