path: improve parse => format combination

make `parse` return an object where `base` is a computed property

Ref: nodejs#1999

Author: refack

lib/path.js

@@ -1114,19 +1114,12 @@ const win32 = {
          startDot === end - 1 &&
          startDot === startPart + 1)) {
       if (end !== -1) {
-        if (startPart === 0 && isAbsolute)
-          ret.base = ret.name = path.slice(rootEnd, end);
-        else
-          ret.base = ret.name = path.slice(startPart, end);
+        const start = (startPart === 0 && isAbsolute) ? rootEnd : startPart;
+        ret.name = path.slice(start, end);
       }
     } else {
-      if (startPart === 0 && isAbsolute) {
-        ret.name = path.slice(rootEnd, startDot);
-        ret.base = path.slice(rootEnd, end);
-      } else {
-        ret.name = path.slice(startPart, startDot);
-        ret.base = path.slice(startPart, end);
-      }
+      const start = (startPart === 0 && isAbsolute) ? rootEnd : startPart;
+      ret.name = path.slice(start, startDot);
       ret.ext = path.slice(startDot, end);
     }
 
@@ -1135,6 +1128,22 @@ const win32 = {
     else if (isAbsolute)
       ret.dir = path.slice(0, rootEnd);
 
+    Object.defineProperty(ret, 'base', {
+      enumerable: true,
+      configurable: false,
+      get() { return this.name + this.ext; },
+      set(value) {
+        const li = value.lastIndexOf('.');
+        if (li > 0) {
+          this.ext = value.slice(li);
+          this.name = value.slice(0, li);
+        } else {
+          this.name = value;
+          this.ext = '';
+        }
+      }
+    });
+
     return ret;
   },
 
@@ -1574,19 +1583,12 @@ const posix = {
          startDot === end - 1 &&
          startDot === startPart + 1)) {
       if (end !== -1) {
-        if (startPart === 0 && isAbsolute)
-          ret.base = ret.name = path.slice(1, end);
-        else
-          ret.base = ret.name = path.slice(startPart, end);
+        const start = (startPart === 0 && isAbsolute) ? 1 : startPart;
+        ret.name = path.slice(start, end);
       }
     } else {
-      if (startPart === 0 && isAbsolute) {
-        ret.name = path.slice(1, startDot);
-        ret.base = path.slice(1, end);
-      } else {
-        ret.name = path.slice(startPart, startDot);
-        ret.base = path.slice(startPart, end);
-      }
+      const start = (startPart === 0 && isAbsolute) ? 1 : startPart;
+      ret.name = path.slice(start, startDot);
       ret.ext = path.slice(startDot, end);
     }
 
@@ -1595,6 +1597,22 @@ const posix = {
     else if (isAbsolute)
       ret.dir = '/';
 
+    Object.defineProperty(ret, 'base', {
+      enumerable: true,
+      configurable: false,
+      get() { return this.name + this.ext; },
+      set(value) {
+        const li = value.lastIndexOf('.');
+        if (li > 0) {
+          this.ext = value.slice(li);
+          this.name = value.slice(0, li);
+        } else {
+          this.name = value;
+          this.ext = '';
+        }
+      }
+    });
+
     return ret;
   },