To create a state object we need to set a state property. We can change this property anytime unlike the props one.

class Header extends React.Component {
    
    constructor(props) {
        super(props);
        this.state = {
            number: 1234
        };
    }
    
    render() {
        return <h2>{this.state.number}</h2>;
    }
};

How to change states?

We shouldn’t change the state object directly. We should use the setState() method instead.

When we don’t depend on the previous state (text typing, for example):

this.setState({
    number: 1235
});

When the previous state is important we use callback function, so it will be called after all previous states are updated.

this.setState(state => ({
    number: 1235
}));

We don’t need to pass to the setState() an object with all the properties. It can be even one property. This property will be overwritten and the rest will be left the same.

What will happen?

Usually after calling the setState() method the render() method will be triggered.

Controlled and uncontrolled inputs

https://goshacmd.com/controlled-vs-uncontrolled-inputs-react/