Follow up: make new warning less wordy

packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js

@@ -215,11 +215,9 @@ describe('ReactComponentLifeCycle', () => {
       ReactTestUtils.renderIntoDocument(<StatefulComponent />);
     }).toWarnDev(
       "Warning: Can't call setState on a component that is not yet mounted. " +
-        'This is a no-op, but it might indicate a bug in your application.\n\n' +
-        'To fix, assign the initial state in the StatefulComponent constructor. ' +
-        'If the state needs to reflect an external data source, ' +
-        'you may also add a componentDidMount lifecycle hook to StatefulComponent ' +
-        'and call setState there if the external data has changed.',
+        'This is a no-op, but it might indicate a bug in your application. ' +
+        'Instead, assign to `this.state` directly or define a `state = {};` ' +
+        'class property with the desired state in the StatefulComponent component.',
     );
 
     // Check deduplication; (no extra warnings should be logged).

packages/react-dom/src/__tests__/ReactCompositeComponent-test.js

@@ -239,11 +239,9 @@ describe('ReactCompositeComponent', () => {
     const container = document.createElement('div');
     expect(() => ReactDOM.render(<MyComponent />, container)).toWarnDev(
       "Warning: Can't call forceUpdate on a component that is not yet mounted. " +
-        'This is a no-op, but it might indicate a bug in your application.\n\n' +
-        'To fix, assign the initial state in the MyComponent constructor. ' +
-        'If the state needs to reflect an external data source, ' +
-        'you may also add a componentDidMount lifecycle hook to MyComponent ' +
-        'and call setState there if the external data has changed.',
+        'This is a no-op, but it might indicate a bug in your application. ' +
+        'Instead, assign to `this.state` directly or define a `state = {};` ' +
+        'class property with the desired state in the MyComponent component.',
     );
 
     // No additional warning should be recorded
@@ -265,11 +263,9 @@ describe('ReactCompositeComponent', () => {
     const container = document.createElement('div');
     expect(() => ReactDOM.render(<MyComponent />, container)).toWarnDev(
       "Warning: Can't call setState on a component that is not yet mounted. " +
-        'This is a no-op, but it might indicate a bug in your application.\n\n' +
-        'To fix, assign the initial state in the MyComponent constructor. ' +
-        'If the state needs to reflect an external data source, ' +
-        'you may also add a componentDidMount lifecycle hook to MyComponent ' +
-        'and call setState there if the external data has changed.',
+        'This is a no-op, but it might indicate a bug in your application. ' +
+        'Instead, assign to `this.state` directly or define a `state = {};` ' +
+        'class property with the desired state in the MyComponent component.',
     );
 
     // No additional warning should be recorded

packages/react/src/ReactNoopUpdateQueue.js

@@ -22,14 +22,11 @@ function warnNoop(publicInstance, callerName) {
     warning(
       false,
       "Can't call %s on a component that is not yet mounted. " +
-        'This is a no-op, but it might indicate a bug in your application.\n\n' +
-        'To fix, assign the initial state in the %s constructor. ' +
-        'If the state needs to reflect an external data source, ' +
-        'you may also add a componentDidMount lifecycle hook to %s ' +
-        'and call setState there if the external data has changed.',
+        'This is a no-op, but it might indicate a bug in your application. ' +
+        'Instead, assign to `this.state` directly or define a `state = {};` ' +
+        'class property with the desired state in the %s component.',
       callerName,
       componentName,
-      componentName,
     );
     didWarnStateUpdateForUnmountedComponent[warningKey] = true;
   }