module: add warning when import,export is detected in CJS context

This will allow users to know how to change their project to support
ES modules.

PR-URL: #28950
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>

lib/internal/modules/cjs/loader.js

@@ -85,6 +85,29 @@ const relativeResolveCache = Object.create(null);
 
 let requireDepth = 0;
 let statCache = null;
+
+function enrichCJSError(err) {
+  const stack = err.stack.split('\n');
+
+  const lineWithErr = stack[1];
+
+  /*
+    The regular expression below targets the most common import statement
+    usage. However, some cases are not matching, cases like import statement
+    after a comment block and/or after a variable definition.
+  */
+  if (err.message.startsWith('Unexpected token export') ||
+    (/^\s*import(?=[ {'"*])\s*(?![ (])/).test(lineWithErr)) {
+    process.emitWarning(
+      'To load an ES module, set "type": "module" in the package.json or use ' +
+      'the .mjs extension.',
+      undefined,
+      undefined,
+      undefined,
+      true);
+  }
+}
+
 function stat(filename) {
   filename = path.toNamespacedPath(filename);
   if (statCache !== null) {
@@ -808,23 +831,32 @@ Module.prototype._compile = function(content, filename) {
       } : undefined,
     });
   } else {
-    const compiled = compileFunction(
-      content,
-      filename,
-      0,
-      0,
-      undefined,
-      false,
-      undefined,
-      [],
-      [
-        'exports',
-        'require',
-        'module',
-        '__filename',
-        '__dirname',
-      ]
-    );
+    let compiled;
+    try {
+      compiled = compileFunction(
+        content,
+        filename,
+        0,
+        0,
+        undefined,
+        false,
+        undefined,
+        [],
+        [
+          'exports',
+          'require',
+          'module',
+          '__filename',
+          '__dirname',
+        ]
+      );
+    } catch (err) {
+      if (experimentalModules) {
+        enrichCJSError(err);
+      }
+      throw err;
+    }
+
     if (experimentalModules) {
       const { callbackMap } = internalBinding('module_wrap');
       callbackMap.set(compiled.cacheKey, {

test/es-module/test-esm-cjs-load-error-note.mjs

@@ -0,0 +1,136 @@
+// Flags: --experimental-modules
+
+import { mustCall } from '../common/index.mjs';
+import assert from 'assert';
+import fixtures from '../common/fixtures.js';
+import { spawn } from 'child_process';
+
+const Export1 = fixtures.path('/es-modules/es-note-unexpected-export-1.cjs');
+const Export2 = fixtures.path('/es-modules/es-note-unexpected-export-2.cjs');
+const Import1 = fixtures.path('/es-modules/es-note-unexpected-import-1.cjs');
+const Import2 = fixtures.path('/es-modules/es-note-promiserej-import-2.cjs');
+const Import3 = fixtures.path('/es-modules/es-note-unexpected-import-3.cjs');
+const Import4 = fixtures.path('/es-modules/es-note-unexpected-import-4.cjs');
+const Import5 = fixtures.path('/es-modules/es-note-unexpected-import-5.cjs');
+
+// Expect note to be included in the error output
+const expectedNote = 'To load an ES module, ' +
+'set "type": "module" in the package.json ' +
+'or use the .mjs extension.';
+
+const expectedCode = 1;
+
+const pExport1 = spawn(process.execPath, ['--experimental-modules', Export1]);
+let pExport1Stderr = '';
+pExport1.stderr.setEncoding('utf8');
+pExport1.stderr.on('data', (data) => {
+  pExport1Stderr += data;
+});
+pExport1.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(pExport1Stderr.includes(expectedNote),
+            `${expectedNote} not found in ${pExport1Stderr}`);
+}));
+
+
+const pExport2 = spawn(process.execPath, ['--experimental-modules', Export2]);
+let pExport2Stderr = '';
+pExport2.stderr.setEncoding('utf8');
+pExport2.stderr.on('data', (data) => {
+  pExport2Stderr += data;
+});
+pExport2.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(pExport2Stderr.includes(expectedNote),
+            `${expectedNote} not found in ${pExport2Stderr}`);
+}));
+// The flag --experimental-modules is not used here
+// the note must not be included in the output
+const pExport3 = spawn(process.execPath, [Export1]);
+let pExport3Stderr = '';
+pExport3.stderr.setEncoding('utf8');
+pExport3.stderr.on('data', (data) => {
+  pExport3Stderr += data;
+});
+pExport3.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(!pExport3Stderr.includes(expectedNote),
+            `${expectedNote} must not be included in ${pExport3Stderr}`);
+}));
+
+const pImport1 = spawn(process.execPath, ['--experimental-modules', Import1]);
+let pImport1Stderr = '';
+pImport1.stderr.setEncoding('utf8');
+pImport1.stderr.on('data', (data) => {
+  pImport1Stderr += data;
+});
+pImport1.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(pImport1Stderr.includes(expectedNote),
+            `${expectedNote} not found in ${pExport1Stderr}`);
+}));
+
+// Note this test shouldn't include the note
+const pImport2 = spawn(process.execPath, ['--experimental-modules', Import2]);
+let pImport2Stderr = '';
+pImport2.stderr.setEncoding('utf8');
+pImport2.stderr.on('data', (data) => {
+  pImport2Stderr += data;
+});
+pImport2.on('close', mustCall((code) => {
+  // As this exits normally we expect 0
+  assert.strictEqual(code, 0);
+  assert.ok(!pImport2Stderr.includes(expectedNote),
+            `${expectedNote} must not be included in ${pImport2Stderr}`);
+}));
+
+const pImport3 = spawn(process.execPath, ['--experimental-modules', Import3]);
+let pImport3Stderr = '';
+pImport3.stderr.setEncoding('utf8');
+pImport3.stderr.on('data', (data) => {
+  pImport3Stderr += data;
+});
+pImport3.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(pImport3Stderr.includes(expectedNote),
+            `${expectedNote} not found in ${pImport3Stderr}`);
+}));
+
+
+const pImport4 = spawn(process.execPath, ['--experimental-modules', Import4]);
+let pImport4Stderr = '';
+pImport4.stderr.setEncoding('utf8');
+pImport4.stderr.on('data', (data) => {
+  pImport4Stderr += data;
+});
+pImport4.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(pImport4Stderr.includes(expectedNote),
+            `${expectedNote} not found in ${pImport4Stderr}`);
+}));
+
+// Must exit with zero and show note
+const pImport5 = spawn(process.execPath, ['--experimental-modules', Import5]);
+let pImport5Stderr = '';
+pImport5.stderr.setEncoding('utf8');
+pImport5.stderr.on('data', (data) => {
+  pImport5Stderr += data;
+});
+pImport5.on('close', mustCall((code) => {
+  assert.strictEqual(code, 0);
+  assert.ok(!pImport5Stderr.includes(expectedNote),
+            `${expectedNote} must not be included in ${pImport5Stderr}`);
+}));
+
+// Must exit with zero and not show note
+const pImport6 = spawn(process.execPath, [Import1]);
+let pImport6Stderr = '';
+pImport6.stderr.setEncoding('utf8');
+pImport6.stderr.on('data', (data) => {
+  pImport6Stderr += data;
+});
+pImport6.on('close', mustCall((code) => {
+  assert.strictEqual(code, expectedCode);
+  assert.ok(!pImport6Stderr.includes(expectedNote),
+            `${expectedNote} must not be included in ${pImport6Stderr}`);
+}));

test/fixtures/es-modules/es-note-promiserej-import-2.cjs

@@ -0,0 +1 @@
+import('valid');

test/fixtures/es-modules/es-note-unexpected-export-1.cjs

@@ -0,0 +1,2 @@
+const example = 10;
+export default example;

test/fixtures/es-modules/es-note-unexpected-export-2.cjs

@@ -0,0 +1,4 @@
+const config = 10;
+export {
+  config
+};

test/fixtures/es-modules/es-note-unexpected-import-1.cjs

@@ -0,0 +1 @@
+import "invalid";

test/fixtures/es-modules/es-note-unexpected-import-3.cjs

@@ -0,0 +1 @@
+import x, { y } from 'xyz'

test/fixtures/es-modules/es-note-unexpected-import-4.cjs

@@ -0,0 +1 @@
+import * as coordinates from 'xyz'

test/fixtures/es-modules/es-note-unexpected-import-5.cjs

@@ -0,0 +1 @@
+import ('invalid')