refactor importer and add tests

src/importers/__tests__/sassTildeImporter.ts

@@ -1,5 +1,5 @@
 import { join } from 'path';
-import { sassTildeImporter } from '../sassTildeImporter';
+import { sassTildeImporter, resolveUrls } from '../sassTildeImporter';
 
 const getAbsoluteFileUrl = (expected: string) =>
   `file://${join(process.cwd(), expected)}`;
@@ -59,6 +59,12 @@ describe('importers / sassTildeImporter', () => {
         })
         ?.toString(),
     ).toBe(getAbsoluteFileUrl('node_modules/bootstrap/scss/_grid.scss'));
+    expect(resolveUrls('~sass-mq/mq.scss')).toContain(
+      'node_modules/sass-mq/_mq.scss',
+    );
+    expect(resolveUrls('~sass-mq/mq')).toContain(
+      'node_modules/sass-mq/_mq.scss',
+    );
   });
 
   it('should resolve index files', () => {

src/importers/sassTildeImporter.ts

@@ -2,70 +2,57 @@ import path from 'path';
 import fs from 'fs';
 import sass from 'sass';
 
+const DEFAULT_EXTS = ['scss', 'sass', 'css'];
+
+export function resolveUrls(url: string, extensions: string[] = DEFAULT_EXTS) {
+  // We only care about tilde-prefixed imports that do not look like paths.
+  if (!url.startsWith('~') || url.startsWith('~/')) {
+    return [];
+  }
+
+  const module_path = path.join('node_modules', url.substring(1));
+  let variants = [module_path];
+
+  const parts = path.parse(module_path);
+
+  // Support sass partials by including paths where the file is prefixed by an underscore.
+  if (!parts.base.startsWith('_')) {
+    const underscore_name = '_'.concat(parts.name);
+    const replacement = {
+      root: parts.root,
+      dir: parts.dir,
+      ext: parts.ext,
+      base: `${underscore_name}${parts.ext}`,
+      name: underscore_name,
+    };
+    variants.push(path.format(replacement));
+  }
+
+  variants.push(path.join(module_path, '_index'));
+
+  // Create subpathsWithExts such that it has entries of the form
+  // node_modules/@foo/bar/baz.(scss|sass)
+  // for an import of the form ~@foo/bar/baz(.(scss|sass))?
+  if (!extensions.some((ext) => parts.ext == `.${ext}`)) {
+    variants = extensions.flatMap((ext) =>
+      variants.map((variant) => `${variant}.${ext}`),
+    );
+  }
+
+  return variants;
+}
+
 /**
  * Creates a sass importer which resolves Webpack-style tilde-imports.
  */
 export const sassTildeImporter: sass.FileImporter<'sync'> = {
   findFileUrl(url) {
-    // We only care about tilde-prefixed imports that do not look like paths.
-    if (!url.startsWith('~') || url.startsWith('~/')) {
-      return null;
-    }
-
-    // Create subpathsWithExts such that it has entries of the form
-    // node_modules/@foo/bar/baz.(scss|sass)
-    // for an import of the form ~@foo/bar/baz(.(scss|sass))?
-    const nodeModSubpath = path.join('node_modules', url.substring(1));
-    const subpathsWithExts: string[] = [];
-    if (
-      nodeModSubpath.endsWith('.scss') ||
-      nodeModSubpath.endsWith('.sass') ||
-      nodeModSubpath.endsWith('.css')
-    ) {
-      subpathsWithExts.push(nodeModSubpath);
-    } else {
-      // Look for .scss first.
-      subpathsWithExts.push(
-        `${nodeModSubpath}.scss`,
-        `${nodeModSubpath}.sass`,
-        `${nodeModSubpath}.css`,
-      );
-    }
-
-    // Support index files.
-    subpathsWithExts.push(
-      `${nodeModSubpath}/_index.scss`,
-      `${nodeModSubpath}/_index.sass`,
-    );
-
-    // Support sass partials by including paths where the file is prefixed by an underscore.
-    const basename = path.basename(nodeModSubpath);
-    if (!basename.startsWith('_')) {
-      const partials = subpathsWithExts.map((file) => {
-        const parts = path.parse(file);
-        const replacement = '_'.concat(parts.name);
-        parts.base = parts.base.replace(parts.name, replacement);
-        parts.name = replacement;
-        return path.format(parts);
-      });
-      subpathsWithExts.push(...partials);
-    }
+    const searchPaths = resolveUrls(url);
 
-    // Climbs the filesystem tree until we get to the root, looking for the first
-    // node_modules directory which has a matching module and filename.
-    let prevDir = '';
-    let dir = path.dirname(url);
-    while (prevDir !== dir) {
-      const searchPaths = subpathsWithExts.map((subpathWithExt) =>
-        path.join(dir, subpathWithExt),
-      );
-      for (const searchPath of searchPaths) {
-        if (fs.existsSync(searchPath)) {
-          return new URL(`file://${path.resolve(searchPath)}`);
-        }
+    for (const searchPath of searchPaths) {
+      if (fs.existsSync(searchPath)) {
+        return new URL(`file://${path.resolve(searchPath)}`);
       }
-      prevDir = dir;
-      dir = path.dirname(dir);
     }
 
     // Returning null is not itself an error, it tells sass to instead try the