Warn if shouldComponentUpdate returns anything other than true/false

src/core/ReactCompositeComponent.js

@@ -1000,9 +1000,22 @@ var ReactCompositeComponentMixin = {
     this._pendingState = null;
 
     try {
-      if (this._pendingForceUpdate ||
-          !this.shouldComponentUpdate ||
-          this.shouldComponentUpdate(nextProps, nextState, nextContext)) {
+      var shouldUpdate =
+        this._pendingForceUpdate ||
+        !this.shouldComponentUpdate ||
+        this.shouldComponentUpdate(nextProps, nextState, nextContext);
+
+      if (__DEV__) {
+        if (typeof shouldUpdate === "undefined") {
+          console.warn(
+            (this.constructor.displayName || 'ReactCompositeComponent') +
+            '.shouldComponentUpdate(): Returned undefined instead of a ' +
+            'boolean value. Make sure to return true or false.'
+          );
+        }
+      }
+
+      if (shouldUpdate) {
         this._pendingForceUpdate = false;
         // Will set `this.props`, `this.state` and `this.context`.
         this._performComponentUpdate(

src/core/__tests__/ReactCompositeComponent-test.js

@@ -871,6 +871,38 @@ describe('ReactCompositeComponent', function() {
     expect(React.isValidClass(TrickFnComponent)).toBe(false);
   });
 
+  it('should warn when shouldComponentUpdate() returns undefined', function() {
+    var warn = console.warn;
+    console.warn = mocks.getMockFunction();
+
+    try {
+      var Component = React.createClass({
+        getInitialState: function () {
+          return {bogus: false};
+        },
+
+        shouldComponentUpdate: function() {
+          return undefined;
+        },
+
+        render: function() {
+          return <div />;
+        }
+      });
+
+      var instance = ReactTestUtils.renderIntoDocument(<Component />);
+      instance.setState({bogus: true});
+
+      expect(console.warn.mock.calls.length).toBe(1);
+      expect(console.warn.mock.calls[0][0]).toBe(
+        'Component.shouldComponentUpdate(): Returned undefined instead of a ' +
+        'boolean value. Make sure to return true or false.'
+      );
+    } finally {
+      console.warn = warn;
+    }
+  });
+
   it('should warn when mispelling shouldComponentUpdate', function() {
     var warn = console.warn;
     console.warn = mocks.getMockFunction();