lnked

<Button
     style={styles.addFotoBtn}
     onPress={this.handleOnPress}
>

handleOnPress = () => {
    Actions.CheckIn(); 
    this.props.onModalOut()
}

class App extends React.Component {
  state = {
    category: 'business',
    items: [],
  };

  componentDidMount() {
    this.fetchData();
  }
  
  fetchData() {
    axios.get('https://newsapi.org/v2/top-headlines?country=us&category='+this.state.category)
      .then(res => {
      this.setState({ items: res.data.articles });
    });
  }
  
  changeCategory = () => {
    this.setState(
      { category: 'entertainment' },
      this.fetchData,
    );
  };


  render() {
    return (
      <div className='wrapper'>
        <button onClick={this.changeCategory}>CLICK</button>
      </div>
    );
  }
}

<Switch>
  <Route path="/auth" component={Auth} />
  <Route path="/" component={Main} />
</Switch>

import libraryName from 'libraryName';

libraryName.someMethod();

import { someMethod } from 'libraryName';

someMethod();

Самому писать

<form>
      <div class="form-group">
        <label for="InputFile">File input</label>
        <input type="file" id="InputFile" @change="onFileChange">
        <p class="help-block">Upload image</p>
      </div>
    </form>

onFileChange (e) {
      let file = e.target.files[0] || e.dataTransfer.files[0]
      let formData = new FormData()
      formData.append('image', file)
      this.$http.post('upload/', formData)
        .then(response => {
          console.log(response)
        }, error => {
          console.log(error)
        })
    }

var re = new RegExp('^https?://(\\w+\\.)?avito\\.ru/([^/]+/)+([a-z_]+(?!_\\d{4,}))$');

var t1 = 'https://www.avito.ru/moskva/odezhda_obuv_aksessuary/magazin_pizham_kigurumi_na_lubyanke_56545';
var t2 = 'https://www.avito.ru/moskva/kvartiry/snimu';

re.test(t1) // false
re.test(t2) // true

import React, { Component } from 'react';
import { withRouter } from 'react-router-dom';

class Example extends Component {
  handler = () => {
    this.props.history.push('/cources');
  };
  
  render() { /* ... */ }
}

export default withRouter(Example);

import React, { Component } from 'react';
import { Link } from 'react-router-dom';

class Example extends Component {
  render() {
    return <Link to="/cources" />;
  }
}

export default Example;

Example extends Component {
  handleClick = e => {
    const { firstHandler, secondHandler } = this.props;

    firstHandler(e);
    secondHander(e);
  };

  render() {
    return <button onClick={this.handleClick}>Click me</button>;
  }
}

<Route
  render = { props => logged ? <Product /> : <Redirect to='/' /> }
  exact
  path='/products/:id'
/>

return state["users"].entities.forEach(user => {
  if (user.email === state["auth"].user.email) {
    return user;
  }
});

return state.users.entities.find(user => user.email === state.auth.user.email);