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Inconsistent behavior of path.basename(path, ext)

I believe this was introduced somewhere in #5123, which changed the behavior of the ext argument.

This is observed on all supported branches and was even recently backported to 4.x.

Documentation:
https://nodejs.org/api/path.html#path_path_basename_path_ext

Observe the input and try to predict the output:

> ['a', 'a/', 'a//'].map(x => path.posix.basename(x))
[ 'a', 'a', 'a' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'b'))
[ 'a', 'a', 'a' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'a'))
[ '', 'a', 'a' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'a/'))
[ 'a', '', 'a' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'a//'))
[ 'a', 'a', '' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'aa'))
[ 'a', 'a/', 'a//' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'bb'))
[ 'a', 'a', 'a' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'aaa'))
[ 'a', 'a', 'a//' ]
> ['a', 'a/', 'a//'].map(x => path.posix.basename(x,'aaaa'))
[ 'a', 'a', 'a' ]
> ['dd', '/dd', 'd/dd', 'd/dd/'].map(x => path.posix.basename(x))
[ 'dd', 'dd', 'dd', 'dd' ]
> ['dd', '/dd', 'd/dd', 'd/dd/'].map(x => path.posix.basename(x, 'd'))
[ 'd', 'd', 'd', 'd' ]
> ['dd', '/dd', 'd/dd', 'd/dd/'].map(x => path.posix.basename(x, 'dd'))
[ '', 'dd', 'dd', 'dd' ]
> ['dd', '/dd', 'd/dd', 'd/dd/'].map(x => path.posix.basename(x, 'ddd'))
[ 'dd', 'dd', 'dd', 'dd/' ]

There are more, but all the inconsistencies with the previous behavior involve at least one of those:

  1. Either the path ends with /,
  2. Or ext includes /,
  3. Or ext equals to the actual resolved basename (i.e. path.endsWith('/' + ext)).

More specifically, the following check covers all the cases inconsistent behavior to my knowledge:
path.endsWith('/') || ext.includes('/') || path.endsWith('/' + ext)
(note that it also includes cases of consistent behavior).

Reminder: before #5123, this was how ext behave:

if (ext && f.substr(-1 * ext.length) === ext) {
    f = f.substr(0, f.length - ext.length);
}

I.e. it just sliced off the suffix (after doing everything else).