path: refactor for performance and consistency

Improve performance by:
+ Modifying an existing array instead of creating a new one
  (this is less readable, but it's a trade-off)
+ Not running the same code twice (in win32.resolve and .normalize)
+ Not leaking the `arguments` object!
+ Getting the last character of a string by index, instead of
  with `.substr()`

Improve code consistency by:
+ Using `[]` instead of `.charAt()` where possible
+ Using a function declaration instead of a var declaration

Improve both by:
+ Using `.slice()` instead of `.substr()`
+ Using `.slice()` with clearer arguments
+ Reusing code with the trimArray() function

lib/path.js

@@ -51,6 +51,27 @@ function normalizeArray(parts, allowAboveRoot) {
   return res;
 }
 
+// returns an array with empty elements removed from either end of the input
+// array or the original array if no elements need to be removed
+function trimArray(arr) {
+  var lastIndex = arr.length - 1;
+  var start = 0;
+  for (; start <= lastIndex; start++) {
+    if (arr[start]) break;
+  }
+
+  var end = lastIndex;
+  for (; end >= 0; end--) {
+    if (arr[end]) break;
+  }
+
+  if (start === 0 && end === lastIndex)
+    return arr;
+  if (start > end)
+    return [];
+  return arr.slice(start, end + 1);
+}
+
 // Regex to split a windows path into three parts: [*, device, slash,
 // tail] windows-only
 var splitDeviceRe =
@@ -69,16 +90,14 @@ function win32SplitPath(filename) {
       device = (result[1] || '') + (result[2] || ''),
       tail = result[3] || '';
   // Split the tail into dir, basename and extension
-  var result2 = splitTailRe.exec(tail),
-      dir = result2[1],
-      basename = result2[2],
-      ext = result2[3];
-  return [device, dir, basename, ext];
+  var retResult = splitTailRe.exec(tail);
+  retResult[0] = device; // Replace the full match with the device
+  return retResult;
 }
 
-var normalizeUNCRoot = function(device) {
+function normalizeUNCRoot(device) {
   return '\\\\' + device.replace(/^[\\\/]+/, '').replace(/[\\\/]+/g, '\\');
-};
+}
 
 // path.resolve([from ...], to)
 win32.resolve = function() {
@@ -115,8 +134,8 @@ win32.resolve = function() {
 
     var result = splitDeviceRe.exec(path),
         device = result[1] || '',
-        isUnc = device && device.charAt(1) !== ':',
-        isAbsolute = win32.isAbsolute(path),
+        isUnc = device && device[1] !== ':',
+        isAbsolute = isUnc || result[2],
         tail = result[3];
 
     if (device &&
@@ -161,8 +180,8 @@ win32.resolve = function() {
 win32.normalize = function(path) {
   var result = splitDeviceRe.exec(path),
       device = result[1] || '',
-      isUnc = device && device.charAt(1) !== ':',
-      isAbsolute = win32.isAbsolute(path),
+      isUnc = device && device[1] !== ':',
+      isAbsolute = isUnc || result[2],
       tail = result[3],
       trailingSlash = /[\\\/]$/.test(tail);
 
@@ -189,20 +208,23 @@ win32.normalize = function(path) {
 win32.isAbsolute = function(path) {
   var result = splitDeviceRe.exec(path),
       device = result[1] || '',
-      isUnc = !!device && device.charAt(1) !== ':';
+      isUnc = !!device && device[1] !== ':';
   // UNC paths are always absolute
-  return !!result[2] || isUnc;
+  return isUnc || !!result[2];
 };
 
 win32.join = function() {
-  function f(p) {
-    if (!util.isString(p)) {
+  var paths = [];
+  for (var i = 0; i < arguments.length; i++) {
+    var arg = arguments[i];
+    if (!util.isString(arg)) {
       throw new TypeError('Arguments to path.join must be strings');
     }
-    return p;
-  }
+    if (arg) {
+      paths.push(arg);
+    }
+  };
 
-  var paths = Array.prototype.filter.call(arguments, f);
   var joined = paths.join('\\');
 
   // Make sure that the joined path doesn't start with two slashes, because
@@ -239,25 +261,10 @@ win32.relative = function(from, to) {
   var lowerFrom = from.toLowerCase();
   var lowerTo = to.toLowerCase();
 
-  function trim(arr) {
-    var start = 0;
-    for (; start < arr.length; start++) {
-      if (arr[start] !== '') break;
-    }
-
-    var end = arr.length - 1;
-    for (; end >= 0; end--) {
-      if (arr[end] !== '') break;
-    }
-
-    if (start > end) return [];
-    return arr.slice(start, end + 1);
-  }
-
-  var toParts = trim(to.split('\\'));
+  var toParts = trimArray(to.split('\\'));
 
-  var lowerFromParts = trim(lowerFrom.split('\\'));
-  var lowerToParts = trim(lowerTo.split('\\'));
+  var lowerFromParts = trimArray(lowerFrom.split('\\'));
+  var lowerToParts = trimArray(lowerTo.split('\\'));
 
   var length = Math.min(lowerFromParts.length, lowerToParts.length);
   var samePartsLength = length;
@@ -320,7 +327,7 @@ win32.dirname = function(path) {
 
   if (dir) {
     // It has a dirname, strip trailing slash
-    dir = dir.substr(0, dir.length - 1);
+    dir = dir.slice(0, -1);
   }
 
   return root + dir;
@@ -360,7 +367,7 @@ win32.format = function(pathObject) {
 
   var dir = pathObject.dir;
   var base = pathObject.base || '';
-  if (dir.slice(dir.length - 1, dir.length) === win32.sep) {
+  if (dir.slice(-1) === win32.sep) {
     return dir + base;
   }
 
@@ -384,7 +391,7 @@ win32.parse = function(pathString) {
   }
   return {
     root: allParts[0],
-    dir: allParts[0] + allParts[1].slice(0, allParts[1].length - 1),
+    dir: allParts[0] + allParts[1].slice(0, -1),
     base: allParts[2],
     ext: allParts[3],
     name: allParts[2].slice(0, allParts[2].length - allParts[3].length)
@@ -425,7 +432,7 @@ posix.resolve = function() {
     }
 
     resolvedPath = path + '/' + resolvedPath;
-    resolvedAbsolute = path.charAt(0) === '/';
+    resolvedAbsolute = path[0] === '/';
   }
 
   // At this point the path should be resolved to a full absolute path, but
@@ -442,7 +449,7 @@ posix.resolve = function() {
 // posix version
 posix.normalize = function(path) {
   var isAbsolute = posix.isAbsolute(path),
-      trailingSlash = path.substr(-1) === '/';
+      trailingSlash = path && path[path.length - 1] === '/';
 
   // Normalize the path
   path = normalizeArray(path.split('/'), !isAbsolute).join('/');
@@ -488,23 +495,8 @@ posix.relative = function(from, to) {
   from = posix.resolve(from).substr(1);
   to = posix.resolve(to).substr(1);
 
-  function trim(arr) {
-    var start = 0;
-    for (; start < arr.length; start++) {
-      if (arr[start] !== '') break;
-    }
-
-    var end = arr.length - 1;
-    for (; end >= 0; end--) {
-      if (arr[end] !== '') break;
-    }
-
-    if (start > end) return [];
-    return arr.slice(start, end + 1);
-  }
-
-  var fromParts = trim(from.split('/'));
-  var toParts = trim(to.split('/'));
+  var fromParts = trimArray(from.split('/'));
+  var toParts = trimArray(to.split('/'));
 
   var length = Math.min(fromParts.length, toParts.length);
   var samePartsLength = length;
@@ -543,7 +535,7 @@ posix.dirname = function(path) {
 
   if (dir) {
     // It has a dirname, strip trailing slash
-    dir = dir.substr(0, dir.length - 1);
+    dir = dir.slice(0, -1);
   }
 
   return root + dir;
@@ -603,7 +595,7 @@ posix.parse = function(pathString) {
 
   return {
     root: allParts[0],
-    dir: allParts[0] + allParts[1].slice(0, allParts[1].length - 1),
+    dir: allParts[0] + allParts[1].slice(0, -1),
     base: allParts[2],
     ext: allParts[3],
     name: allParts[2].slice(0, allParts[2].length - allParts[3].length)