fix: explain why RHL could not be activated, fixes #1362

index.js

@@ -13,18 +13,28 @@ if (process.env.NODE_ENV === 'production') {
   module.exports.hot.shouldWrapWithAppContainer = true;
 } else {
   var evalAllowed = false;
+  var evalError = null;
   try {
     eval('evalAllowed = true');
   } catch (e) {
     // eval not allowed due to CSP
+    evalError = e && e.message ? e.message : 'unknown reason';
   }
 
+  // TODO: dont use eval to update methods. see #1273
   // RHL needs setPrototypeOf to operate Component inheritance, and eval to patch methods
   var jsFeaturesPresent = !!Object.setPrototypeOf;
 
   if (!jsFeaturesPresent || !evalAllowed) {
     // we are not in prod mode, but RHL could not be activated
-    console.warn('React-Hot-Loader is not supported in this environment.');
+    console.warn(
+      'React-Hot-Loader is not supported in this environment:',
+      [
+        !jsFeaturesPresent && "some JS features are missing",
+        !evalAllowed && "`eval` is not allowed(" + evalError + ")"
+      ].join(','),
+      '.'
+    );
     module.exports = require('./dist/react-hot-loader.production.min.js');
   } else {
     module.exports = window.reactHotLoaderGlobal = require('./dist/react-hot-loader.development.js');