Как объединить элемент в последний массив внутри корневого массива?

class MyApp extends React.Component {
  constructor() {
    super();
    this.interval;
    this.state = {arr: []};
    this._getFakeCoords = this._getFakeCoords.bind(this);
    this._startStop = this._startStop.bind(this);
  }
  
  _getFakeCoords() {   
    return {lat: Math.random() * 100, long: Math.random() * 100};
  }
  
  _startStop() {
    if(this.interval) {
      clearInterval(this.interval);
      this.interval = null;
    } else {
      let newArr = this.state.arr.slice();  //copy the array so we can mutate it
      this.interval = setInterval(() => {
        newArr.push([this._getFakeCoords(), this._getFakeCoords()]);
        this.setState({arr: newArr});  //replace the old array with the mutated one
        console.log(newArr);  //preview the result
      }, 3000);
    }
  }
  
  render() {
    return (
      <div>
      	<button onClick={this._startStop}>Start/Stop</button>
      </div>
    );
  }
}

ReactDOM.render(<MyApp />, document.getElementById("app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>