Migrating from jscpd

Fallow can replace jscpd when you want duplication detection as part of a broader codebase audit. It also covers unused code detection, complexity analysis, circular dependencies, and architecture boundaries in the same binary.

Migration workflow

1. 1

   Preview the migration

   Run fallow migrate --dry-run to see the generated config without writing anything:

   fallow migrate --dry-run

   Fallow searches for .jscpd.json and the jscpd field in package.json.

2. 2

   Generate the config

   Once you're happy with the preview, run the migration:

   fallow migrate            # Write .fallowrc.json
   fallow migrate --toml     # Or output as fallow.toml

3. 3

   Review the config

   Open the generated config and verify the translated thresholds, token counts, and ignore patterns.

4. 4

   Run fallow dupes

   Run your first duplication analysis:

   fallow dupes

What gets migrated

jscpd	fallow
threshold	duplicates.threshold
minTokens	duplicates.minTokens
minLines	duplicates.minLines
ignore	ignorePatterns
format	-- (fallow auto-detects languages)

Comparison

	fallow	jscpd
Raw duplication speed	Part of the broader audit flow	jscpd v5 is faster on the current README benchmark fixtures
Detection modes	4 (strict, mild, weak, semantic)	3 (strict, mild, weak)
Algorithm	Suffix array with LCP	Rust rewrite in v5
Unused code + circular deps	Built-in (fallow dead-code)	Not included
Complexity + maintainability	Built-in (fallow health)	Not included
Clone families	Yes, with refactoring suggestions	No
Cross-language	TS to JS type stripping	Format-based
Baseline tracking	Yes	No
Runtime dependency	Native binary	Native binary in v5

Detection mode mapping

jscpd v5 supports strict, mild, and weak modes. The closest fallow equivalent for exact token matching is strict.

Fallow defaults to mild mode, which normalizes syntax variations and may flag more clones than jscpd did. To match jscpd's behavior more closely, use strict mode:

fallow dupes --mode strict

For broader matching that catches renamed variables:

fallow dupes --mode semantic

One tool for the full picture

One binary replaces separate jscpd, knip, and madge workflows:

fallow dead-code    # Unused code + circular deps + boundaries
fallow dupes        # Code duplication (replaces jscpd)
fallow health       # Complexity hotspots + maintainability scores
fallow              # All of the above in one pass

See also