Reagent port of official React tutorial code upto "Lifting State Up" https://facebook.github.io/react/tutorial/tutorial.html#lifting-state-up

react-tutorial-v2.core.cljs

(ns react-tutorial.core
  (:require
    [reagent.core :as r]))

(defn Square [value click-fn]
  [:button.square {:on-click click-fn} value])

(defn Board []
  (let [squares (r/atom (apply vector (repeat 9 nil)))
        handle-click #(swap! squares assoc % "X")
        renderSquare (fn [i] 
                       [Square (get @squares i) #(handle-click i)])
        status (r/atom "Next player: X")]
    (fn []
      [:div
       [:div.status @status]
       [:div.board-row
        [renderSquare 0]
        [renderSquare 1]
        [renderSquare 2]]
       [:div.board-row
        [renderSquare 3]
        [renderSquare 4]
        [renderSquare 5]]
       [:div.board-row
        [renderSquare 6]
        [renderSquare 7]
        [renderSquare 8]]])))

(defn Game []
  [:div.game
   [:div.game-board
    [Board]]
   [:div.game-info
    [:div]
    [:ol]]])

(r/render
  [Game]
  (js/document.getElementById "container"))


(defn calculateWinner [squares]
  (let [lines [[0 1 2]
               [3 4 5]
               [6 7 8]
               [0 3 6]
               [1 4 7]
               [2 5 8]
               [0 4 8]
               [2 4 6]]]
    (->> lines
         (filter (fn [[a b c]]
                   (and
                     (not (nil? (get squares a)))
                     (= (get squares a)
                        (get squares b)
                        (get squares c)))))
         first
         first
         (get squares))))

react-tutorial-v2.js-equivalent.js

class Square extends React.Component {
  render() {
    return (
      <button className="square"  onClick={() => this.props.onClick()}>
        {this.props.value}
      </button>
    );
  }
}

class Board extends React.Component {
  constructor() {
    super();
    this.state = {
      squares: Array(9).fill(null),
    };
  }
  handleClick(i) {
    const squares = this.state.squares.slice();
    squares[i] = 'X';
    this.setState({squares: squares});
  }
  renderSquare(i) {
    return <Square value={this.state.squares[i]} onClick={() => this.handleClick(i)} />;  
  }
  render() {
    const status = 'Next player: X';
    return (
      <div>
        <div className="status">{status}</div>
        <div className="board-row">
          {this.renderSquare(0)}
          {this.renderSquare(1)}
          {this.renderSquare(2)}
        </div>
        <div className="board-row">
          {this.renderSquare(3)}
          {this.renderSquare(4)}
          {this.renderSquare(5)}
        </div>
        <div className="board-row">
          {this.renderSquare(6)}
          {this.renderSquare(7)}
          {this.renderSquare(8)}
        </div>
      </div>
    );
  }
}

class Game extends React.Component {
  render() {
    return (
      <div className="game">
        <div className="game-board">
          <Board />
        </div>
        <div className="game-info">
          <div>{/* status */}</div>
          <ol>{/* TODO */}</ol>
        </div>
      </div>
    );
  }
}


// ========================================

ReactDOM.render(
  <Game />,
  document.getElementById('container')
);

function calculateWinner(squares) {
  const lines = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6],
  ];
  for (let i = 0; i < lines.length; i++) {
    const [a, b, c] = lines[i];
    if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
      return squares[a];
    }
  }
  return null;
}