feat: fall back to module resolution when relative imports fail

src/loaders/sass/importer.ts

@@ -1,41 +1,60 @@
 import path from "path";
-import { packageFilterBuilder, resolveAsync, resolveSync } from "../../utils/resolve";
-import { getUrlOfPartial, isModule, normalizeUrl } from "../../utils/url";
+import { isAbsolutePath, isRelativePath } from "../../utils/path";
+import { packageFilterBuilder, resolveAsync, ResolveOpts, resolveSync } from "../../utils/resolve";
+import { getUrlOfPartial, hasModuleSpecifier, normalizeUrl } from "../../utils/url";
 
 const extensions = [".scss", ".sass", ".css"];
 const conditions = ["sass", "style"];
 
-export const importer: sass.Importer = (url, importer, done): void => {
-  const finalize = (id: string): void => done({ file: id.replace(/\.css$/i, "") });
-  const next = (): void => done(null);
+/**
+ * The exact behavior of importers defined here differ slightly between dart-sass and node-sass:
+ * https://github.com/sass/dart-sass/issues/574
+ *
+ * In short, dart-sass specifies that the *correct* behavior is to only call importers when a
+ * stylesheet fails to resolve via relative path. Since these importers below are implementation-
+ * agnostic, the first attempt to resolve a file by a relative is unneeded in dart-sass and can be
+ * removed once support for node-sass is fully deprecated.
+ */
+function importerImpl<T extends (ids: string[], userOpts: ResolveOpts) => unknown>(
+  url: string,
+  importer: string,
+  resolve: T,
+): ReturnType<T> {
+  const candidates: string[] = [];
+  if (hasModuleSpecifier(url)) {
+    const moduleUrl = normalizeUrl(url);
+    // Give precedence to importing a partial
+    candidates.push(getUrlOfPartial(moduleUrl), moduleUrl);
+  } else {
+    const relativeUrl = normalizeUrl(url);
+    candidates.push(getUrlOfPartial(relativeUrl), relativeUrl);
 
-  if (!isModule(url)) return next();
-  const moduleUrl = normalizeUrl(url);
-  const partialUrl = getUrlOfPartial(moduleUrl);
+    // fall back to module imports
+    if (!isAbsolutePath(url) && !isRelativePath(url)) {
+      const moduleUrl = normalizeUrl(`~${url}`);
+      candidates.push(getUrlOfPartial(moduleUrl), moduleUrl);
+    }
+  }
   const options = {
     caller: "Sass importer",
     basedirs: [path.dirname(importer)],
     extensions,
     packageFilter: packageFilterBuilder({ conditions }),
   };
-  // Give precedence to importing a partial
-  resolveAsync([partialUrl, moduleUrl], options).then(finalize).catch(next);
-};
+  return resolve(candidates, options) as ReturnType<T>;
+}
 
 const finalize = (id: string): sass.Data => ({ file: id.replace(/\.css$/i, "") });
+
+export const importer: sass.Importer = (url, importer, done): void => {
+  void importerImpl(url, importer, resolveAsync)
+    .then(id => done(finalize(id)))
+    .catch(() => done(null));
+};
+
 export const importerSync: sass.Importer = (url, importer): sass.Data => {
-  if (!isModule(url)) return null;
-  const moduleUrl = normalizeUrl(url);
-  const partialUrl = getUrlOfPartial(moduleUrl);
-  const options = {
-    caller: "Sass importer",
-    basedirs: [path.dirname(importer)],
-    extensions,
-    packageFilter: packageFilterBuilder({ conditions }),
-  };
-  // Give precedence to importing a partial
   try {
-    return finalize(resolveSync([partialUrl, moduleUrl], options));
+    return finalize(importerImpl(url, importer, resolveSync));
   } catch {
     return null;
   }

src/utils/url.ts

@@ -1,15 +1,15 @@
 import path from "path";
 import { isAbsolutePath, isRelativePath, normalizePath } from "./path";
 
-export const isModule = (url: string): boolean => /^~[\d@A-Za-z]/.test(url);
+export const hasModuleSpecifier = (url: string): boolean => /^~[\d@A-Za-z]/.test(url);
 
 export function getUrlOfPartial(url: string): string {
   const { dir, base } = path.parse(url);
   return dir ? `${normalizePath(dir)}/_${base}` : `_${base}`;
 }
 
 export function normalizeUrl(url: string): string {
-  if (isModule(url)) return normalizePath(url.slice(1));
+  if (hasModuleSpecifier(url)) return normalizePath(url.slice(1));
   if (isAbsolutePath(url) || isRelativePath(url)) return normalizePath(url);
   return `./${normalizePath(url)}`;
 }