Incrementing timer in React, in JavaScript and ClojureScript (Reagent)

Increment (Plain React).js

var Timer = React.createClass({
  getInitialState: function() {
    return {secondsElapsed: 0};
  },
  tick: function() {
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});
  },
  componentDidMount: function() {
    this.interval = setInterval(this.tick, 1000);
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  render: function() {
    return (
      <div>Seconds Elapsed: {this.state.secondsElapsed}</div>
    );
  }
});

React.render(<Timer />, mountNode);

Increment (Reagent).cljs

;; State
(def app-state (atom 0)) ;; Initial value is 0

;; Increment seconds
(js/setInterval #(swap! app-state inc) 1000) ;; Increment app-state every 1000ms

;; "Seconds elapsed" component 
(defn timer []
  [:div (str "Seconds elapsed: " @app-state)])

(render-component [timer] (get-element :body)) ;; Render it