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: https://github.com/nodejs/node/pull/28950
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>

1 files changed, 48 insertions, 18 deletions

diff --git a/lib/internal/modules/cjs/loader.js b/lib/internal/modules/cjs/loader.js
index 5d5767e0cf..3d440202c2 100644
--- a/lib/internal/modules/cjs/loader.js
+++ b/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) {
@@ -828,24 +851,31 @@ function wrapSafe(filename, content) {
       } : undefined,
     });
   }
-
-  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');