path.relative(dir, dir) returns empty string

[eush77]

path.relative returns empty string when its from and to arguments are identical.

> path.relative(dir, dir)
''

Since '' is not a valid path, this can blow up:

function ls (dir) {
  dir = path.relative(process.cwd(), dir);
  console.log(dir);
  // ...

  console.log(fs.readdirSync(dir));
}

This function ls works most of the time (nonexistent files and permissions aside), but not if dir points to the working directory:

> ls('.')

Error: ENOENT: no such file or directory, scandir ''
    at Error (native)
    at Object.fs.readdirSync (fs.js:856:18)
    at ls (repl:6:16)

I find the fact that path.relative returns a valid path, except in one corner case when the two arguments are identical, rather inconvenient.

However, this is probably intentional since there is a test for this introduced in #2106. But I haven't found any reasoning for this, and the reasoning of #2106 itself is not (directly) applicable here: it's one thing to defensively treat empty strings as '.', and the other to return an empty string instead of a valid path.

Expected bahavior:

> path.relative(dir, dir)
'.'

[silverwind]

I see what you mean, but '.' means 'current directory' to me, something which does not seem correct for me for arbitrary paths which are equal. It would only be correct when dir equals $PWD.

[eush77]

I see what you mean, but '.' means 'current directory' to me, something which does not seem correct for me for arbitrary paths which are equal. It would only be correct when dir equals $PWD.

That's not actually correct. Every directory has '.' and '..', and they can appear anywhere in the path:

$ cd /tmp
$ cd ../var/./opt/../../.
$ pwd
/

[zeusdeux]

Shouldn't path.relative(dir, dir) just return dir?

[silverwind]

Shouldn't path.relative(dir, dir) just return dir?

No, that would imply there's another sub-directory like /dir/dir when dir = '/dir'.

That's not actually correct. Every directory has '.' and '..', and they can appear anywhere in the path:

Right, I see returning '.' has some uses, like process.chdir(path.relative(process.cwd(), 'dir')). Still, I'm uncomfortable with breaking documented behaviour.

[evanlucas]

This is documented at https://nodejs.org/dist/latest-v5.x/docs/api/path.html#path_path_relative_from_to.

I'll go ahead and close. Thanks!