Why is the output different with React setState()

Question

I don't know why

• When I use this.setState({count: count+1} Clicking multiple times will only update count once
• When I use this.setState({count: this.setState.count}) Each click updates the count
• Why different?

       class Demo extends React.Component {
            state = {count: 0};
            render() {
                const {count} = this.state;
                return  <button onClick={() => this.setState({count: count+1})}>count</button>
            }
            shouldComponentUpdate(){
                return false;
            } 

        }

• Environment [email protected]