module: check file ext before dir as documented

The documented resolution algorithm started to search for package.json
files prior to searching for file extensions when searching for a
specifier. Oddly, it did not search for index files at same time it
searched for package.json. This restores the documented behavior of
searching for file extensions prior to searching directories.

PR-URL: #15015
Fixes: #14990
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>

lib/module.js

@@ -219,6 +219,9 @@ Module._findPath = function(request, paths, isMain) {
   var exts;
   var trailingSlash = request.length > 0 &&
     request.charCodeAt(request.length - 1) === CHAR_FORWARD_SLASH;
+  if (!trailingSlash) {
+    trailingSlash = /(?:^|\/)\.?\.$/.test(request);
+  }
 
   // For each path
   for (var i = 0; i < paths.length; i++) {
@@ -236,10 +239,6 @@ Module._findPath = function(request, paths, isMain) {
         } else {
           filename = toRealPath(basePath);
         }
-      } else if (rc === 1) {  // Directory.
-        if (exts === undefined)
-          exts = Object.keys(Module._extensions);
-        filename = tryPackage(basePath, exts, isMain);
       }
 
       if (!filename) {
@@ -251,11 +250,13 @@ Module._findPath = function(request, paths, isMain) {
     }
 
     if (!filename && rc === 1) {  // Directory.
+      // try it with each of the extensions at "index"
       if (exts === undefined)
         exts = Object.keys(Module._extensions);
-      filename = tryPackage(basePath, exts, isMain) ||
-        // try it with each of the extensions at "index"
-        tryExtensions(path.resolve(basePath, 'index'), exts, isMain);
+      filename = tryPackage(basePath, exts, isMain);
+      if (!filename) {
+        filename = tryExtensions(path.resolve(basePath, 'index'), exts, isMain);
+      }
     }
 
     if (filename) {

test/fixtures/module-extension-over-directory/inner.js

@@ -0,0 +1 @@
+module.exports = {};

test/fixtures/module-extension-over-directory/inner/package.json

@@ -0,0 +1,3 @@
+{
+  "main": "./package.json"
+}

test/parallel/test-require-extension-over-directory.js

@@ -0,0 +1,30 @@
+'use strict';
+// fixes regression from v4
+require('../common');
+const assert = require('assert');
+const fixtures = require('../common/fixtures');
+const path = require('path');
+
+const fixturesRequire = require(
+  fixtures.path('module-extension-over-directory', 'inner'));
+
+assert.strictEqual(
+  fixturesRequire,
+  require(fixtures.path('module-extension-over-directory', 'inner.js')),
+  'test-require-extension-over-directory failed to import fixture' +
+  ' requirements'
+);
+
+const fakePath = [
+  fixtures.path('module-extension-over-directory', 'inner'),
+  'fake',
+  '..'
+].join(path.sep);
+const fixturesRequireDir = require(fakePath);
+
+assert.strictEqual(
+  fixturesRequireDir,
+  require(fixtures.path('module-extension-over-directory', 'inner/')),
+  'test-require-extension-over-directory failed to import fixture' +
+  ' requirements'
+);