path: using .relative() should not return a trailing slash

Resolving a path against root with `path.relative()` should not
include a trailing slash.

Fixes: https://github.com/nodejs/node/issues/28549

PR-URL: https://github.com/nodejs/node/pull/28556
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/lib/path.js b/lib/path.js
index 2301a6ebb83..f7a8612933a 100644
--- a/lib/path.js
+++ b/lib/path.js
@@ -1090,11 +1090,16 @@ const posix = {
           // For example: from='/'; to='/foo'
           return to.slice(toStart + i);
         }
-      } else if (fromLen > length &&
-                 from.charCodeAt(fromStart + i) === CHAR_FORWARD_SLASH) {
-        // We get here if `to` is the exact base path for `from`.
-        // For example: from='/foo/bar/baz'; to='/foo/bar'
-        lastCommonSep = i;
+      } else if (fromLen > length) {
+        if (from.charCodeAt(fromStart + i) === CHAR_FORWARD_SLASH) {
+          // We get here if `to` is the exact base path for `from`.
+          // For example: from='/foo/bar/baz'; to='/foo/bar'
+          lastCommonSep = i;
+        } else if (i === 0) {
+          // We get here if `to` is the root.
+          // For example: from='/foo/bar'; to='/'
+          lastCommonSep = 0;
+        }
       }
     }