lnked

this.setState(state => Object
  .keys(state)
  .reduce((acc, n) => (acc[n] = n === 'свойствоКотороеДолжноБытьTrue', acc), {})
);

<Fragment>
  <Modal />
  <Router />
</Fragment>

сначала сделать запрос <...> после чего отрисовать страницу

state = {
  active: null,
}

toggle = ({ target: { dataset: { name } } }) => {
  this.setState(({ active }) => ({
    active: active === name ? null : name,
  }));
}

<button onClick={this.toggle} data-name="collapse_1"></button>
<button onClick={this.toggle} data-name="collapse_2"></button>
<button onClick={this.toggle} data-name="collapse_3"></button>

toggle = ({ target: { dataset: { name } } }) => {
  this.setState(({ collapse }) => ({
    collaple: {
      ...collapse,
      [name]: !collapse[name],
    },
  }));
}

class Hello extends React.Component {
  componentWillUnmount() {
    if (this.timer) {
      clearTimeout(this.timer);
    }
  }

  someMethod() {
    return new Promise((resolve) => {
      this.timer = setTimeout(resolve, 5000);
    });
  }
}

class ExampleComponent extends React.Component {
  state = {
    isReady: false,
  };
  
  hanldeOnLoad = () => {
    this.setState({ isReady: true });
  };
  
  render() {
    const { src } = this.props;
    const { isReady } = this.state;
 
    const className = isReady ? '' : 'some-class';
 
    return (
      <div className={className}>
        <img onLoad={this.hanldeOnLoad} src={src} />
      </div>
    );
  }
}

const Grid extends React.Component {
  state = { itemsSize: 'sm' };

  handleChangeSize = () => { /* some code */ };  

  render() {
    const { itemSize } = this.state;
    const { items } = this.props;

    return (
      <Wrapper>
        <ControlPanel onChangeSize={this.handleChangeSize} />
        {items.map(item => <Item key={item.id} size={this.state.itemSize} />)}
      </Wrapper>
    );
  }
}