jscpd
Measures the share of duplicated lines in a codebase and shows where. Runs with no install through npx. MIT licence.
Strengths
- No install: npx jscpd is enough
- Language independent, over 220 formats recognised
Limitations
- The percentage alone does not say whether a duplicate is a flaw: a copied test is normal
- Two major versions coexist (v4 and v5): pin the one you want
Best for
- Measuring, before and after an agent session, how much code was copied