Improve error message when mounting non-string/function elements

src/isomorphic/classic/element/__tests__/ReactElementValidator-test.js

@@ -327,17 +327,18 @@ describe('ReactElementValidator', function() {
     });
     expect(function() {
       ReactTestUtils.renderIntoDocument(React.createElement(ParentComp));
-    }).toThrow();
-    expect(console.error.calls.length).toBe(2);
+    }).toThrow(
+      'Invariant Violation: Element type is invalid: expected a string (for ' +
+      'built-in components) or a class/function (for composite components) ' +
+      'but got: null. Check the render method of `ParentComp`.'
+    );
+    expect(console.error.calls.length).toBe(1);
     expect(console.error.calls[0].args[0]).toBe(
       'Warning: React.createElement: type should not be null, undefined, ' +
       'boolean, or number. It should be a string (for DOM elements) or a ' +
       'ReactClass (for composite components). Check the render method of ' +
       '`ParentComp`.'
     );
-    expect(console.error.calls[1].args[0]).toBe(
-      'Warning: Only functions or strings can be mounted as React components.'
-    );
   });
 
   it('should check default prop values', function() {

src/renderers/shared/reconciler/__tests__/ReactComponent-test.js

@@ -279,4 +279,32 @@ describe('ReactComponent', function() {
     );
   });
 
+  it('throws usefully when rendering badly-typed elements', function() {
+    spyOn(console, 'error');
+
+    var X = undefined;
+    expect(() => ReactTestUtils.renderIntoDocument(<X />)).toThrow(
+      'Invariant Violation: Element type is invalid: expected a string (for ' +
+      'built-in components) or a class/function (for composite components) ' +
+      'but got: undefined.'
+    );
+
+    var Y = null;
+    expect(() => ReactTestUtils.renderIntoDocument(<Y />)).toThrow(
+      'Invariant Violation: Element type is invalid: expected a string (for ' +
+      'built-in components) or a class/function (for composite components) ' +
+      'but got: null.'
+    );
+
+    var Z = {};
+    expect(() => ReactTestUtils.renderIntoDocument(<Z />)).toThrow(
+      'Invariant Violation: Element type is invalid: expected a string (for ' +
+      'built-in components) or a class/function (for composite components) ' +
+      'but got: object.'
+    );
+
+    // One warning for each element creation
+    expect(console.error.calls.length).toBe(3);
+  });
+
 });

src/renderers/shared/reconciler/instantiateReactComponent.js

@@ -30,6 +30,16 @@ assign(
   }
 );
 
+function getDeclarationErrorAddendum(owner) {
+  if (owner) {
+    var name = owner.getName();
+    if (name) {
+      return ' Check the render method of `' + name + '`.';
+    }
+  }
+  return '';
+}
+
 /**
  * Check if the type reference is a known internal type. I.e. not a user
  * provided composite type.
@@ -63,13 +73,14 @@ function instantiateReactComponent(node, parentCompositeType) {
 
   if (typeof node === 'object') {
     var element = node;
-    if (__DEV__) {
-      warning(
-        element && (typeof element.type === 'function' ||
-                    typeof element.type === 'string'),
-        'Only functions or strings can be mounted as React components.'
-      );
-    }
+    invariant(
+      element && (typeof element.type === 'function' ||
+                  typeof element.type === 'string'),
+      'Element type is invalid: expected a string (for built-in components) ' +
+      'or a class/function (for composite components) but got: %s.%s',
+      element.type == null ? element.type : typeof element.type,
+      getDeclarationErrorAddendum(element._owner)
+    );
 
     // Special case string values
     if (parentCompositeType === element.type &&