[Performance] use isDirectory to speed up node_modules lookups

This is a backport of 4cf8928 and
fa11d48 (#190 and #191) to the 1.x branch.

This offers a small but useful performance improvement by avoiding unnecessary stat calls.

Author: keithamus

lib/async.js

@@ -219,8 +219,14 @@ module.exports = function resolve(x, options, callback) {
         if (dirs.length === 0) return cb(null, undefined);
         var dir = dirs[0];
 
-        var file = path.join(dir, x);
-        loadAsFile(file, opts.package, onfile);
+        isDirectory(dir, isdir);
+
+        function isdir(err, isdir) {
+            if (err) return cb(err);
+            if (!isdir) return processDirs(cb, dirs.slice(1));
+            var file = path.join(dir, x);
+            loadAsFile(file, opts.package, onfile);
+        }
 
         function onfile(err, m, pkg) {
             if (err) return cb(err);

lib/sync.js

@@ -156,10 +156,12 @@ module.exports = function (x, options) {
         var dirs = nodeModulesPaths(start, opts, x);
         for (var i = 0; i < dirs.length; i++) {
             var dir = dirs[i];
-            var m = loadAsFileSync(path.join(dir, '/', x));
-            if (m) return m;
-            var n = loadAsDirectorySync(path.join(dir, '/', x));
-            if (n) return n;
+            if (isDirectory(dir)) {
+                var m = loadAsFileSync(path.join(dir, '/', x));
+                if (m) return m;
+                var n = loadAsDirectorySync(path.join(dir, '/', x));
+                if (n) return n;
+            }
         }
     }
 };

test/mock.js

@@ -108,6 +108,7 @@ test('mock package', function (t) {
 
     var dirs = {};
     dirs[path.resolve('/foo')] = true;
+    dirs[path.resolve('/foo/node_modules')] = true;
 
     function opts(basedir) {
         return {
@@ -142,6 +143,7 @@ test('mock package from package', function (t) {
 
     var dirs = {};
     dirs[path.resolve('/foo')] = true;
+    dirs[path.resolve('/foo/node_modules')] = true;
 
     function opts(basedir) {
         return {

test/mock_sync.js

@@ -56,6 +56,7 @@ test('mock package', function (t) {
 
     var dirs = {};
     dirs[path.resolve('/foo')] = true;
+    dirs[path.resolve('/foo/node_modules')] = true;
 
     function opts(basedir) {
         return {