Allow external CommonJS modules to be imported properly in Node

esinstall/src/index.ts

@@ -373,7 +373,8 @@ ${colors.dim(
         warning.code === 'CIRCULAR_DEPENDENCY' ||
         warning.code === 'NAMESPACE_CONFLICT' ||
         warning.code === 'THIS_IS_UNDEFINED' ||
-        warning.code === 'EMPTY_BUNDLE'
+        warning.code === 'EMPTY_BUNDLE' ||
+        warning.code === 'UNUSED_EXTERNAL_IMPORT'
       ) {
         logger.debug(logMessage);
       } else {

snowpack/assets/require-or-import.js

@@ -20,16 +20,28 @@ module.exports = async function requireOrImport(filepath, { from = process.cwd()
         });
         try {
           const mdl = NATIVE_REQUIRE(resolvedPath);
+
+          // Add a `default` property on a CommonJS export so that it can be accessed from import statements.
+          // This is set to enumerable: false to make it hidden(ish) to code.
+          if(mdl && !('default' in mdl)) {
+            Object.defineProperty(mdl, 'default', {
+              configurable: true,
+              enumerable: false,
+              writable: true,
+              value: mdl
+            });
+          }
+
           resolve(mdl);
         } catch (e) {
           if (e instanceof SyntaxError && /export|import/.test(e.message)) {
             console.error(`Failed to load "${filepath}"!\nESM format is not natively supported in "node@${process.version}".\nPlease use CommonJS or upgrade to an LTS version of node above "node@12.17.0".`)
           } else if (e.code === 'ERR_REQUIRE_ESM') {
               const url = pathToFileURL(resolvedPath);
-              return NATIVE_IMPORT(url).then(mdl => resolve(mdl.default ? mdl.default : mdl));
-          };
+              return NATIVE_IMPORT(url).then(mdl => resolve(mdl.default ? mdl.default : mdl), err => reject(err));
+          }
           try {
-            return NATIVE_IMPORT(pathToFileURL(resolvedPath)).then(mdl => resolve(mdl.default ? mdl.default : mdl));
+            return NATIVE_IMPORT(pathToFileURL(resolvedPath)).then(mdl => resolve(mdl.default ? mdl.default : mdl), err => reject(err));
           } catch (e) {
             reject(e);  
           }

test/snowpack/runtime/runtime.test.js

@@ -90,4 +90,49 @@ describe('runtime', () => {
       await fixture.cleanup();
     }
   });
+
+  it('Can import a CommonJS module as the default export', async () => {
+    const fixture = await testRuntimeFixture({
+      'packages/other/package.json': dedent`
+        {
+          "version": "1.0.0",
+          "name": "other",
+          "main": "main.js"
+        }
+      `,
+      'packages/other/main.js': dedent`
+        module.exports = () => 'works';
+      `,
+      'main.js': dedent`
+        import fn from 'other';
+
+        export function test() {
+          return fn();
+        }
+      `,
+      'package.json': dedent`
+        {
+          "version": "1.0.1",
+          "name": "@snowpack/test-runtime-import-cjs",
+          "dependencies": {
+            "other": "file:./packages/other"
+          }
+        }
+      `,
+      'snowpack.config.json': dedent`
+        {
+          "packageOptions": {
+            "external": ["other"]
+          }
+        }
+      `
+    });
+
+    try {
+      let mod = await fixture.runtime.importModule('/main.js');
+      expect(await mod.exports.test()).toEqual('works');
+    } finally {
+      await fixture.cleanup();
+    }
+  });
 });