React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

.red {
  border: 3px solid red;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

React

class TodoApp extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
    	items: [
      	{ id:1, text: "Learn JavaScript", done: false },
        { id:2, text: "Learn React", done: false },
        { id:3, text: "Play around in JSFiddle", done: true },
        { id:4, text: "Build something awesome", done: true }
      ]
    }
    this.addItem = this.addItem.bind(this);
    this.removeItem = this.removeItem.bind(this);
  }
  
  addItem() {
  	let newItemVal = this.refs.newItem.value;
   	let newObj = { id:5, text: newItemVal, done: false}
   	let items = this.state.items;
    
    if(this.refs.newItem.value == '') {
    	this.refs.newItem.classList.add("red");
      this.refs.newItem.focus();
      return;
    }
    
    items.push(newObj);
    this.setState( 
    	items
    );
    
    this.refs.newItem.classList.remove("red");
    this.refs.newItem.value = '';
    this.refs.load.textContent = "";
    this.refs.newItem.focus();
  }
  
  removeItem(i) {
  	this.refs.newItem.classList.add("red");
  }
  
  componentDidMount() {
  	this.refs.newItem.focus();
    this.refs.load.textContent = "Загрузка...";
    
  }
  
  render() {
    return (
      <div>
        <h2>Todos:</h2>
        <ol>
        {this.state.items.map(item => (
          <li key={item.id}>
            <label>
              <input type="checkbox" disabled readOnly checked={item.done} /> 
              <span className={item.done ? "done" : ""}>{item.text}</span>
             
            </label>
             <span onClick={this.removeItem(item.id)}> x</span>
          </li>
        ))}
        </ol>
        <input type="text" ref="newItem"></input>
        <button onClick={this.addItem}>Add new item</button>
        <span ref = "load"></span>
      </div>
    )
  }
}

ReactDOM.render(<TodoApp />, document.querySelector("#app"))