path: refactor path joining logic for clarity and performance

Author: wlgh1553

lib/path.js

@@ -506,21 +506,20 @@ const win32 = {
     if (args.length === 0)
       return '.';
 
-    let joined;
-    let firstPart;
+    const path = [];
     for (let i = 0; i < args.length; ++i) {
       const arg = args[i];
       validateString(arg, 'path');
       if (arg.length > 0) {
-        if (joined === undefined)
-          joined = firstPart = arg;
-        else
-          joined += `\\${arg}`;
+        path.push(arg);
       }
     }
 
-    if (joined === undefined)
+    if (path.length === 0)
       return '.';
+    
+    const firstPart = path[0];
+    let joined = ArrayPrototypeJoin(path, '\\');
 
     // Make sure that the joined path doesn't start with two slashes, because
     // normalize() will mistake it for a UNC path then.