React State Summary

Update state with setState, except in constructor

this.setState({
  title: 'Updated title',
});

Set state with previous state

this.setState((prevState, props) => {
  return {count: prevState.count + 1};
});

Declare initial state in constructor

class Heading extends React.Component {
  constructor(props) {
    super(props);
    this.state = {title: 'My title'};
  }
}

Do not update state directly

this.state.title = 'Hello';

Lifting state up to share state between component

class Wrapper extends React.Component {
  constructor(props) {
    super(props);
    this.handleInputChange = this.handleInputChange.bind(this);
    this.state = {value: ''};
  }

  handleInputChange(value) {
    this.setState({value});
  }
  render() {
    const value = this.state.value;
    return (
        <Input value={value} onInputChange={this.handleInputChange} />
    );
  }
}

class Input extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
  }

  handleChange(e) {
    this.props.onInputChange(e.target.value);
  }

  render() {
    const value = this.props.value;
    return <input value={value} onChange={this.handleChange} />;
  }
}

Source: React Cheat Sheet