const rootElement = document.getElementById("root");

const state = {
  cities: ["Montreal", "Toronto", "Vancouver"],
  index: undefined,
}

function setState(nextState) {
  Object.assign(state, nextState);
  render();
}

function render() {
const { cities, index } = state;
const element = (
  <div>
    <p>You will travel to: { cities[index] }</p>
    <button onClick={
        (event) => {
          setState({
            index: Math.floor(Math.random()*cities.length)
          });
        }
      }>
      Pick a city
    </button>
  </div>
);

ReactDOM.render(element, rootElement);
}
render();