Improve not-yet-mounted setState warning (#12531)

* Tweak not-yet-mounted setState warning

* Add \n\n

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

@@ -214,10 +214,12 @@ describe('ReactComponentLifeCycle', () => {
     expect(() => {
       ReactTestUtils.renderIntoDocument(<StatefulComponent />);
     }).toWarnDev(
-      'Warning: setState(...): Can only update a mounted or ' +
-        'mounting component. This usually means you called setState() on an ' +
-        'unmounted component. This is a no-op.\n\nPlease check the code for the ' +
-        'StatefulComponent component.',
+      "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.',
     );
 
     // Check deduplication; (no extra warnings should be logged).

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

@@ -225,6 +225,58 @@ describe('ReactCompositeComponent', () => {
     expect(inputProps.prop).not.toBeDefined();
   });
 
+  it('should warn about `forceUpdate` on not-yet-mounted components', () => {
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        this.forceUpdate();
+      }
+      render() {
+        return <div />;
+      }
+    }
+
+    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.',
+    );
+
+    // No additional warning should be recorded
+    const container2 = document.createElement('div');
+    ReactDOM.render(<MyComponent />, container2);
+  });
+
+  it('should warn about `setState` on not-yet-mounted components', () => {
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        this.setState();
+      }
+      render() {
+        return <div />;
+      }
+    }
+
+    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.',
+    );
+
+    // No additional warning should be recorded
+    const container2 = document.createElement('div');
+    ReactDOM.render(<MyComponent />, container2);
+  });
+
   it('should warn about `forceUpdate` on unmounted components', () => {
     const container = document.createElement('div');
     document.body.appendChild(container);

packages/react/src/ReactNoopUpdateQueue.js

@@ -21,12 +21,15 @@ function warnNoop(publicInstance, callerName) {
     }
     warning(
       false,
-      '%s(...): Can only update a mounted or mounting component. ' +
-        'This usually means you called %s() on an unmounted component. ' +
-        'This is a no-op.\n\nPlease check the code for the %s component.',
-      callerName,
+      "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.',
       callerName,
       componentName,
+      componentName,
     );
     didWarnStateUpdateForUnmountedComponent[warningKey] = true;
   }