test: add test suite for getWebpackResolver

Because `getWebpackResolver` is now part of `utils.js`' public API, we
should test it separately so that we have some protection against its
API changing drastically from potential future refactors.

package.json

@@ -77,6 +77,7 @@
     "css-loader": "^4.2.0",
     "del": "^5.1.0",
     "del-cli": "^3.0.1",
+    "enhanced-resolve": "^4.3.0",
     "eslint": "^7.6.0",
     "eslint-config-prettier": "^6.11.0",
     "eslint-plugin-import": "^2.21.2",

test/resolver.test.js

@@ -0,0 +1,36 @@
+import { fileURLToPath } from 'url';
+
+import enhanced from 'enhanced-resolve';
+
+import { getWebpackResolver } from '../src/utils';
+
+/**
+ * Because `getWebpackResolver` is a public function that can be imported by
+ * external packages, we want to test it separately to ensure its API does not
+ * change unexpectedly.
+ */
+describe('getWebpackResolver', () => {
+  const resolve = (request, ...options) =>
+    getWebpackResolver(enhanced.create, ...options)(__filename, request);
+
+  it('should resolve .scss from node_modules', async () => {
+    expect(await resolve('scss/style')).toMatch(/style\.scss$/);
+  });
+
+  it('should resolve from passed `includePaths`', async () => {
+    expect(await resolve('empty', null, [`${__dirname  }/scss`])).toMatch(
+      /empty\.scss$/
+    );
+  });
+
+  it('should reject when file cannot be resolved', async () => {
+    await expect(resolve('foo/bar/baz')).rejects.toBe();
+  });
+
+  it('should strip an invalid file URL of its scheme', async () => {
+    const invalidFileURL = 'file://scss/empty';
+
+    expect(() => fileURLToPath(invalidFileURL)).toThrow();
+    expect(await resolve(invalidFileURL)).toMatch(/empty\.scss$/);
+  });
+});