Improve error messages for invalid element types

cc @mkonicek https://twitter.com/martinkonicek/status/810917317639110661

scripts/fiber/tests-passing.txt

@@ -286,7 +286,7 @@ src/isomorphic/classic/element/__tests__/ReactElementValidator-test.js
 * does not warn when the element is directly in rest args
 * does not warn when the array contains a non-element
 * should give context for PropType errors in nested components.
-* gives a helpful error when passing null, undefined, boolean, or number
+* gives a helpful error when passing invalid types
 * should check default prop values
 * should not check the default for explicit null
 * should check declared prop types

src/isomorphic/classic/element/ReactElementValidator.js

@@ -191,13 +191,30 @@ var ReactElementValidator = {
     // We warn in this case but don't throw. We expect the element creation to
     // succeed and there will likely be errors in render.
     if (!validType) {
-      warning(
-        false,
-        '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).%s',
-        getDeclarationErrorAddendum()
-      );
+      if (
+        typeof type !== 'function' &&
+        typeof type !== 'string'
+      ) {
+        var info = '';
+        if (
+          typeof type === 'object' &&
+          type !== null &&
+          Object.keys(type).length === 0
+        ) {
+          info +=
+            ' You likely forgot to export your component from the file ' +
+            'it\'s defined in.';
+        }
+        info += getDeclarationErrorAddendum();
+        warning(
+          false,
+          'React.createElement: type is invalid -- expected a string (for ' +
+          'built-in components) or a class/function (for composite ' +
+          'components) but got: %s.%s',
+          type == null ? type : typeof type,
+          info,
+        );
+      }
     }
 
     var element = ReactElement.createElement.apply(this, arguments);

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

@@ -286,35 +286,48 @@ describe('ReactElementValidator', () => {
     );
   });
 
-  it('gives a helpful error when passing null, undefined, boolean, or number', () => {
+  it('gives a helpful error when passing invalid types', () => {
     spyOn(console, 'error');
     React.createElement(undefined);
     React.createElement(null);
     React.createElement(true);
     React.createElement(123);
-    expectDev(console.error.calls.count()).toBe(4);
+    React.createElement({x: 17});
+    React.createElement({});
+    expectDev(console.error.calls.count()).toBe(6);
     expectDev(console.error.calls.argsFor(0)[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).'
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: undefined.'
     );
     expectDev(console.error.calls.argsFor(1)[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).'
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: null.'
     );
     expectDev(console.error.calls.argsFor(2)[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).'
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: boolean.'
     );
     expectDev(console.error.calls.argsFor(3)[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).'
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: number.'
+    );
+    expectDev(console.error.calls.argsFor(4)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: object.'
+    );
+    expectDev(console.error.calls.argsFor(5)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: object. You likely forgot to export your ' +
+      'component from the file it\'s defined in.'
     );
     React.createElement('div');
-    expectDev(console.error.calls.count()).toBe(4);
+    expectDev(console.error.calls.count()).toBe(6);
   });
 
   it('includes the owner name when passing null, undefined, boolean, or number', () => {
@@ -333,10 +346,9 @@ describe('ReactElementValidator', () => {
     );
     expectDev(console.error.calls.count()).toBe(1);
     expectDev(console.error.calls.argsFor(0)[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`.'
+      'Warning: React.createElement: 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`.'
     );
   });
 
@@ -534,9 +546,9 @@ describe('ReactElementValidator', () => {
     void <Foo>{[<div />]}</Foo>;
     expectDev(console.error.calls.count()).toBe(1);
     expectDev(console.error.calls.argsFor(0)[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).'
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: undefined.'
     );
   });
 

src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js

@@ -256,21 +256,25 @@ describe('ReactJSXElementValidator', () => {
     void <True />;
     void <Num />;
     expectDev(console.error.calls.count()).toBe(4);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
-      'type should not be null, undefined, boolean, or number. It should be ' +
-      'a string (for DOM elements) or a ReactClass (for composite components).'
+    expectDev(console.error.calls.argsFor(0)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: undefined.'
     );
-    expectDev(console.error.calls.argsFor(1)[0]).toContain(
-      'type should not be null, undefined, boolean, or number. It should be ' +
-      'a string (for DOM elements) or a ReactClass (for composite components).'
+    expectDev(console.error.calls.argsFor(1)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: null.'
     );
-    expectDev(console.error.calls.argsFor(2)[0]).toContain(
-      'type should not be null, undefined, boolean, or number. It should be ' +
-      'a string (for DOM elements) or a ReactClass (for composite components).'
+    expectDev(console.error.calls.argsFor(2)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: boolean.'
     );
-    expectDev(console.error.calls.argsFor(3)[0]).toContain(
-      'type should not be null, undefined, boolean, or number. It should be ' +
-      'a string (for DOM elements) or a ReactClass (for composite components).'
+    expectDev(console.error.calls.argsFor(3)[0]).toBe(
+      'Warning: React.createElement: type is invalid -- expected a string ' +
+      '(for built-in components) or a class/function (for composite ' +
+      'components) but got: number.'
     );
     void <Div />;
     expectDev(console.error.calls.count()).toBe(4);

src/renderers/shared/fiber/ReactFiber.js

@@ -334,7 +334,11 @@ function createFiberFromElementType(type : mixed, key : null | string) : Fiber {
   } else if (typeof type === 'string') {
     fiber = createFiber(HostComponent, key);
     fiber.type = type;
-  } else if (typeof type === 'object' && type !== null) {
+  } else if (
+    typeof type === 'object' &&
+    type !== null &&
+    typeof type.tag === 'number'
+  ) {
     // Currently assumed to be a continuation and therefore is a fiber already.
     // TODO: The yield system is currently broken for updates in some cases.
     // The reified yield stores a fiber, but we don't know which fiber that is;
@@ -343,12 +347,25 @@ function createFiberFromElementType(type : mixed, key : null | string) : Fiber {
     // There is probably a clever way to restructure this.
     fiber = ((type : any) : Fiber);
   } else {
+    let info = '';
+    if (__DEV__) {
+      if (
+        typeof type === 'object' &&
+        type !== null &&
+        Object.keys(type).length === 0
+      ) {
+        info +=
+          ' You likely forgot to export your component from the file ' +
+          'it\'s defined in.';
+      }
+    }
+    // TODO: Stack also includes owner name in the message.
     invariant(
       false,
       'Element type is invalid: expected a string (for built-in components) ' +
-      'or a class/function (for composite components) but got: %s.',
+      'or a class/function (for composite components) but got: %s.%s',
       type == null ? type : typeof type,
-      // TODO: Stack also includes owner name in the message.
+      info,
     );
   }
   return fiber;

src/renderers/shared/stack/reconciler/instantiateReactComponent.js

@@ -72,14 +72,32 @@ function instantiateReactComponent(node, shouldHaveDebugID) {
     instance = ReactEmptyComponent.create(instantiateReactComponent);
   } else if (typeof node === 'object') {
     var element = node;
-    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)
-    );
+    var type = element.type;
+    if (
+      typeof type !== 'function' &&
+      typeof type !== 'string'
+    ) {
+      var info = '';
+      if (__DEV__) {
+        if (
+          typeof type === 'object' &&
+          type !== null &&
+          Object.keys(type).length === 0
+        ) {
+          info +=
+            ' You likely forgot to export your component from the file ' +
+            'it\'s defined in.';
+        }
+      }
+      info += getDeclarationErrorAddendum(element._owner);
+      invariant(
+        false,
+        'Element type is invalid: expected a string (for built-in components) ' +
+        'or a class/function (for composite components) but got: %s.%s',
+        type == null ? type : typeof type,
+        info,
+      );
+    }
 
     // Special case string values
     if (typeof element.type === 'string') {