fs: minor refactoring

1. Removed a few unnecessary variables to reduce LoC

2. Removed redundant `var` definitions of variables in same function

3. Refactored variables which are defined inside a block and used
outside as well

4. Refactored effect-less code

5. In `rethrow` function, instead of assigning to `err` and throwing
`err`, we can directly throw `backtrace` object itself.

6. Reassigning a defined parameter while also mentioning arguments in
the body is one of the optimization killers. So, changing `callback` to
`callback_` and declaring a new variable called `callback` in the body.

lib/fs.js

@@ -49,8 +49,7 @@ function rethrow() {
       if (err) {
         backtrace.stack = err.name + ': ' + err.message +
                           backtrace.stack.substr(backtrace.name.length);
-        err = backtrace;
-        throw err;
+        throw backtrace;
       }
     };
   }
@@ -267,27 +266,25 @@ function ReadFileContext(callback, encoding) {
 }
 
 ReadFileContext.prototype.read = function() {
-  var fd = this.fd;
-  var size = this.size;
   var buffer;
   var offset;
   var length;
 
-  if (size === 0) {
+  if (this.size === 0) {
     buffer = this.buffer = new SlowBuffer(kReadFileBufferLength);
     offset = 0;
     length = kReadFileBufferLength;
   } else {
     buffer = this.buffer;
     offset = this.pos;
-    length = size - this.pos;
+    length = this.size - this.pos;
   }
 
   var req = new FSReqWrap();
   req.oncomplete = readFileAfterRead;
   req.context = this;
 
-  binding.read(fd, buffer, offset, length, -1, req);
+  binding.read(this.fd, buffer, offset, length, -1, req);
 };
 
 ReadFileContext.prototype.close = function(err) {
@@ -302,8 +299,7 @@ function readFileAfterOpen(err, fd) {
   var context = this.context;
 
   if (err) {
-    var callback = context.callback;
-    callback(err);
+    context.callback(err);
     return;
   }
 
@@ -417,7 +413,7 @@ fs.readFileSync = function(path, options) {
   if (size === 0) {
     buffers = [];
   } else {
-    var threw = true;
+    threw = true;
     try {
       buffer = new Buffer(size);
       threw = false;
@@ -427,16 +423,18 @@ fs.readFileSync = function(path, options) {
   }
 
   var done = false;
+  var bytesRead;
+
   while (!done) {
-    var threw = true;
+    threw = true;
     try {
       if (size !== 0) {
-        var bytesRead = fs.readSync(fd, buffer, pos, size - pos);
+        bytesRead = fs.readSync(fd, buffer, pos, size - pos);
       } else {
         // the kernel lies about many files.
         // Go ahead and try to read some bytes.
         buffer = new Buffer(8192);
-        var bytesRead = fs.readSync(fd, buffer, 0, 8192);
+        bytesRead = fs.readSync(fd, buffer, 0, 8192);
         if (bytesRead) {
           buffers.push(buffer.slice(0, bytesRead));
         }
@@ -529,8 +527,8 @@ function modeNum(m, def) {
   return undefined;
 }
 
-fs.open = function(path, flags, mode, callback) {
-  callback = makeCallback(arguments[arguments.length - 1]);
+fs.open = function(path, flags, mode, callback_) {
+  var callback = makeCallback(arguments[arguments.length - 1]);
   mode = modeNum(mode, 0o666);
 
   if (!nullCheck(path, callback)) return;
@@ -585,10 +583,12 @@ fs.read = function(fd, buffer, offset, length, position, callback) {
 
 fs.readSync = function(fd, buffer, offset, length, position) {
   var legacy = false;
+  var encoding;
+
   if (!(buffer instanceof Buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     legacy = true;
-    var encoding = arguments[3];
+    encoding = arguments[3];
 
     assertEncoding(encoding);
 
@@ -623,14 +623,14 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     callback(err, written || 0, buffer);
   }
 
+  var req = new FSReqWrap();
   if (buffer instanceof Buffer) {
     // if no position is passed then assume null
     if (typeof position === 'function') {
       callback = position;
       position = null;
     }
     callback = maybeCallback(callback);
-    var req = new FSReqWrap();
     req.oncomplete = strWrapper;
     return binding.writeBuffer(fd, buffer, offset, length, position, req);
   }
@@ -647,7 +647,6 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     length = 'utf8';
   }
   callback = maybeCallback(position);
-  var req = new FSReqWrap();
   req.oncomplete = bufWrapper;
   return binding.writeString(fd, buffer, offset, length, req);
 };
@@ -721,8 +720,10 @@ fs.truncateSync = function(path, len) {
   }
   // allow error to be thrown, but still close fd.
   var fd = fs.openSync(path, 'r+');
+  var ret;
+
   try {
-    var ret = fs.ftruncateSync(fd, len);
+    ret = fs.ftruncateSync(fd, len);
   } finally {
     fs.closeSync(fd);
   }
@@ -875,7 +876,7 @@ function preprocessSymlinkDestination(path, type, linkPath) {
   }
 }
 
-fs.symlink = function(destination, path, type_, callback) {
+fs.symlink = function(destination, path, type_, callback_) {
   var type = (typeof type_ === 'string' ? type_ : null);
   var callback = makeCallback(arguments[arguments.length - 1]);
 
@@ -968,9 +969,9 @@ if (constants.hasOwnProperty('O_SYMLINK')) {
 
     // prefer to return the chmod error, if one occurs,
     // but still try to close, and report closing errors if they occur.
-    var err, err2;
+    var err, err2, ret;
     try {
-      var ret = fs.fchmodSync(fd, mode);
+      ret = fs.fchmodSync(fd, mode);
     } catch (er) {
       err = er;
     }
@@ -1088,8 +1089,8 @@ fs.futimesSync = function(fd, atime, mtime) {
   binding.futimes(fd, atime, mtime);
 };
 
-function writeAll(fd, buffer, offset, length, position, callback) {
-  callback = maybeCallback(arguments[arguments.length - 1]);
+function writeAll(fd, buffer, offset, length, position, callback_) {
+  var callback = maybeCallback(arguments[arguments.length - 1]);
 
   // write(fd, buffer, offset, length, position, callback)
   fs.write(fd, buffer, offset, length, position, function(writeErr, written) {
@@ -1112,7 +1113,7 @@ function writeAll(fd, buffer, offset, length, position, callback) {
   });
 }
 
-fs.writeFile = function(path, data, options, callback) {
+fs.writeFile = function(path, data, options, callback_) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
   if (!options || typeof options === 'function') {
@@ -1627,8 +1628,8 @@ function ReadStream(path, options) {
   this.flags = options.flags === undefined ? 'r' : options.flags;
   this.mode = options.mode === undefined ? 0o666 : options.mode;
 
-  this.start = options.start === undefined ? undefined : options.start;
-  this.end = options.end === undefined ? undefined : options.end;
+  this.start = options.start;
+  this.end = options.end;
   this.autoClose = options.autoClose === undefined ? true : options.autoClose;
   this.pos = undefined;
 
@@ -1790,7 +1791,7 @@ function WriteStream(path, options) {
   this.flags = options.flags === undefined ? 'w' : options.flags;
   this.mode = options.mode === undefined ? 0o666 : options.mode;
 
-  this.start = options.start === undefined ? undefined : options.start;
+  this.start = options.start;
   this.pos = undefined;
   this.bytesWritten = 0;