Styjun

this is the first time I face this warning message.

Can't call setState on a component that is not yet mounted.

Follows:

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.

The "not yet mounted" part actually makes little to no sense as the only way to trigger the issue is to call a function by clicking a button from a component that needs to be mounted in order to see the button. The component is not unmounted at any given time neither.

This dummy component reproduces the error in my app:

import PropTypes from 'prop-types'
import React from 'react'

export default class MyComponent extends React.Component 
 constructor (props) 
 super(props)
 this.state = 
 initial: 'state'
 
 this.clickMe = this.clickMe.bind(this)
 

 clickMe () 
 this.setState(
 some: 'new state'
 )
 

 render () 
 return (
 <div>
 <button onClick=this.clickMe>click</button>
 </div>
 )
 

I am using:

"react": "16.3.2",
"react-dom": "16.3.2",
"mobx": "4.2.0",
"mobx-react": "5.1.2",

Did I miss something in the latest React/mobx version? (note the component does not use any mobx related stuff but its parent is a mobx-react observer)

Edit:

There must be something related to the component instance, further investigation has shown that in some cases, creating an handler inside the render function will make this warning disappear, but not in all cases.

class MyComponent extends React.component 
 constructor (props) 
 // ...
 this.clickMeBound = this.clickMe.bind(this)
 

 clickMe () 
 ...
 

 render () 
 // works
 <button onClick=() => this.clickMe()>click arrow in render</button>

 // warning: Can't call setState on a component that is not yet mounted.
 <button onClick=this.clickMeBound>click bound</button>
 

Edit 2:

I have removed 'react-hot-loader/patch' from my entries in my Webpack config and some weird issues like this one have disappeared. I'm not putting this as an answer because the error message itself is still weird and this causes a warning in the console. Everything works fine though.

Just add this line of code to the code

export default class MyComponent extends React.Component 
 constructor (props) 
 super(props)
 this.state = 
 initial: 'state',
 some: '' // <------- this line
 
 this.clickMe = this.clickMe.bind(this)
 

 clickMe () 
 this.setState(
 some: 'new state'
 )
 

 render () 
 return (
 <div>
 <button onClick=this.clickMe>click</button>
 </div>
 );
 

As @Amida mentioned, hot-loader seems to be the issue. Whoever is using

app.UseWebpackDevMiddleware(new WebpackDevMiddlewareOptions

 HotModuleReplacement = true,
 ReactHotModuleReplacement = true
);

in Startup.cs, remove it and the issue will disappear. I don't know why, but this is my current workaround.

EDIT:

Update of "react-hot-loader" and "webpack-hot-middleware" to latest versions fixed the issue

If this error is happening in one of your tests, you might need to render the component to an element before accessing it (i.e. simply doing let app = new App; is not enough). Rendering will effectively mount the component and its children, as explained in this other answer and then you will be able to use the result object to perform operations without triggering the setState error. A simple App.test.js example:

import App from './App';

it('renders without crashing', () => 
 const div = document.createElement('div');
 ReactDOM.render(<App />, div); // <-- App component mounted here
 // without JSX: ReactDOM.render(React.createElement(App), div)
 ReactDOM.unmountComponentAtNode(div);
);

test('array length decreased after removal', () => 
 const div = document.createElement('div');
 let app = ReactDOM.render(<App />, div); // <-- App component mounted here
 const origArrLen = app.state.arr.length;

 app.removeAtIndex(0);
 expect(app.state.arr.length).toEqual(origArrLen - 1);
 ReactDOM.unmountComponentAtNode(div);
);

Where the App component could have:

class App extends Component 
 state = 
 arr: [1,2,3]
 ;

 removeAtIndex = index => 
 const arr = this.state;
 this.setState( arr: arr.filter((el, i) => i !== index) );
 ;
 // render() return ( ... )