dirname appears not to work with xargs

xargs and basename work together as I would expect: $ printf '%s\n' foo/index.js bar/index.js baz/index.js | xargs basename index.js index.js index.js xargs and dirname, though, appear not to work together: $ printf '%s\n' foo/index.js bar/index.js baz/index.js | xargs dirname usage: dirname path I would expect foo bar baz as output. What am I missing? I'm on Darwin 18.2.0 (macOS 10.14.3).