Move importsToResolve to dedicated module

And refactor imperative code in favor of a functional approach

lib/importsToResolve.js

@@ -0,0 +1,49 @@
+"use strict";
+
+const path = require("path");
+
+// libsass uses this precedence when importing files without extension
+const extPrecedence = [".scss", ".sass", ".css"];
+
+/**
+ * When libsass tries to resolve an import, it uses a special algorithm.
+ * Since the sass-loader uses webpack to resolve the modules, we need to simulate that algorithm. This function
+ * returns an array of import paths to try.
+ *
+ * @param {string} request
+ * @returns {Array}
+ */
+function importsToResolve(request) {
+    // libsass' import algorithm works like this:
+    // In case there is no file extension...
+    //   - Prefer modules starting with '_'.
+    //   - File extension precedence: .scss, .sass, .css.
+    // In case there is a file extension...
+    //   - If the file is a CSS-file, do not include it all, but just link it via @import url().
+    //   - The exact file name must match (no auto-resolving of '_'-modules).
+
+    // Keep in mind: ext can also be something like '.datepicker' when the true extension is omitted and the filename contains a dot.
+    // @see https://github.com/jtangelder/sass-loader/issues/167
+    const ext = path.extname(request);
+    const basename = path.basename(request);
+    const dirname = path.dirname(request);
+    const startsWithUnderscore = basename.charAt(0) === "_";
+    // a module import is an identifier like 'bootstrap-sass'
+    // We also need to check for dirname since it might also be a deep import like 'bootstrap-sass/something'
+    const isModuleImport = request.charAt(0) !== "." && dirname === ".";
+    const hasCssExt = ext === ".css";
+    const hasSassExt = ext === ".scss" || ext === ".sass";
+
+    return (isModuleImport && [request]) || // Do not modify module imports
+        (hasCssExt && []) || // Do not import css files
+        (hasSassExt && [request]) || // Do not modify imports with explicit extensions
+        (startsWithUnderscore ? [] : extPrecedence) // Do not add underscore imports if there is already an underscore
+            .map(ext => "_" + basename + ext)
+            .concat(
+                extPrecedence.map(ext => basename + ext)
+            ).map(
+                file => dirname + "/" + file // No path.sep required here, because imports inside SASS are usually with /
+            );
+}
+
+module.exports = importsToResolve;

lib/loader.js

@@ -7,10 +7,9 @@ const os = require("os");
 const async = require("async");
 const assign = require("object-assign");
 const formatSassError = require("./formatSassError");
+const importsToResolve = require("./importsToResolve");
 
-// libsass uses this precedence when importing files without extension
 const slice = Array.prototype.slice;
-const extPrecedence = [".scss", ".sass", ".css"];
 const matchCss = /\.css$/;
 
 // This queue makes sure node-sass leaves one thread available for executing
@@ -47,7 +46,7 @@ function sassLoader(content) {
             const request = utils.urlToRequest(url, sassOptions.root);
             const dirContext = fileToDirContext(fileContext);
 
-            resolve(dirContext, url, getImportsToResolve(request), done);
+            resolve(dirContext, url, importsToResolve(request), done);
         };
     }
 
@@ -191,64 +190,6 @@ function sassLoader(content) {
     });
 }
 
-/**
- * When libsass tries to resolve an import, it uses a special algorithm.
- * Since the sass-loader uses webpack to resolve the modules, we need to simulate that algorithm. This function
- * returns an array of import paths to try.
- *
- * @param {string} originalImport
- * @returns {Array}
- */
-function getImportsToResolve(originalImport) {
-    // libsass' import algorithm works like this:
-    // In case there is no file extension...
-    //   - Prefer modules starting with '_'.
-    //   - File extension precedence: .scss, .sass, .css.
-    // In case there is a file extension...
-    //   - If the file is a CSS-file, do not include it all, but just link it via @import url().
-    //   - The exact file name must match (no auto-resolving of '_'-modules).
-    const ext = path.extname(originalImport);
-    const basename = path.basename(originalImport);
-    const dirname = path.dirname(originalImport);
-    const startsWithUnderscore = basename.charAt(0) === "_";
-    const paths = [];
-
-    function add(file) {
-        // No path.sep required here, because imports inside SASS are usually with /
-        paths.push(dirname + "/" + file);
-    }
-
-    if (originalImport.charAt(0) !== ".") {
-        // If true: originalImport is a module import like 'bootstrap-sass...'
-        if (dirname === ".") {
-            // If true: originalImport is just a module import without a path like 'bootstrap-sass'
-            // In this case we don't do that auto-resolving dance at all.
-            return [originalImport];
-        }
-    }
-
-    // We can't just check for ext being defined because ext can also be something like '.datepicker'
-    // when the true extension is omitted and the filename contains a dot.
-    // @see https://github.com/jtangelder/sass-loader/issues/167
-    if (ext === ".css") {
-        // do not import css files
-    } else if (ext === ".scss" || ext === ".sass") {
-        add(basename);
-    } else {
-        if (!startsWithUnderscore) {
-            // Prefer modules starting with '_' first
-            extPrecedence.forEach((ext) => {
-                add("_" + basename + ext);
-            });
-        }
-        extPrecedence.forEach((ext) => {
-            add(basename + ext);
-        });
-    }
-
-    return paths;
-}
-
 /**
  * Check the loader query and webpack config for loader options. If an option is defined in both places,
  * the loader query takes precedence.