Move proxyCustomImporters to dedicated module

lib/loader.js

@@ -8,8 +8,8 @@ const async = require("async");
 const assign = require("object-assign");
 const formatSassError = require("./formatSassError");
 const importsToResolve = require("./importsToResolve");
+const proxyCustomImporters = require("./proxyCustomImporters");
 
-const slice = Array.prototype.slice;
 const matchCss = /\.css$/;
 
 // This queue makes sure node-sass leaves one thread available for executing
@@ -207,33 +207,4 @@ function getLoaderConfig(loaderContext) {
     return assign({}, config, query);
 }
 
-/**
- * Creates new custom importers that use the given `resourcePath` if libsass calls the custom importer with `prev`
- * being 'stdin'.
- *
- * Why do we need this? We have to use the `data` option of node-sass in order to compile our sass because
- * the `resourcePath` might not be an actual file on disk. When using the `data` option, libsass uses the string
- * 'stdin' instead of a filename.
- *
- * We have to fix this behavior in order to provide a consistent experience to the webpack user.
- *
- * @param {function|Array<function>} importer
- * @param {string} resourcePath
- * @returns {Array<function>}
- */
-function proxyCustomImporters(importer, resourcePath) {
-    return [].concat(importer).map((importer) => {
-        return function (url, prev, done) {
-            const importerContext = this; // eslint-disable-line no-invalid-this
-            const args = slice.call(arguments);
-
-            if (args[1] === "stdin") {
-                args[1] = resourcePath;
-            }
-
-            return importer.apply(importerContext, args);
-        };
-    });
-}
-
 module.exports = sassLoader;

lib/proxyCustomImporters.js

@@ -0,0 +1,29 @@
+"use strict";
+
+/**
+ * Creates new custom importers that use the given `resourcePath` if libsass calls the custom importer with `prev`
+ * being 'stdin'.
+ *
+ * Why do we need this? We have to use the `data` option of node-sass in order to compile our sass because
+ * the `resourcePath` might not be an actual file on disk. When using the `data` option, libsass uses the string
+ * 'stdin' instead of a filename.
+ *
+ * We have to fix this behavior in order to provide a consistent experience to the webpack user.
+ *
+ * @param {function|Array<function>} importer
+ * @param {string} resourcePath
+ * @returns {Array<function>}
+ */
+function proxyCustomImporters(importer, resourcePath) {
+    return [].concat(importer).map((importer) => {
+        return function (url, prev, done) {
+            return importer.apply(
+                this, // eslint-disable-line no-invalid-this
+                Array.from(arguments)
+                    .map((arg, i) => i === 1 && arg === "stdin" ? resourcePath : arg)
+            );
+        };
+    });
+}
+
+module.exports = proxyCustomImporters;