renderActions.js

class Game extends Compoent {
...  
  renderActions() {
    const { history } = this.state;
    const selectedCell = this.getSelectedCell();
    const prefilled = selectedCell && selectedCell.get('prefilled');
    return (
      <div className="actions">
        <div className="action" onClick={history.size ? this.undo : null}>
          <ReloadIcon />Undo
        </div>
        <div className="action redo" onClick={history.size ? this.redo : null}>
          <ReloadIcon />Redo
        </div>
        <div className="action" onClick={!prefilled ? this.eraseSelected : null}>
          <RemoveIcon />Erase
        </div>
        <div
          className="action"
          onClick={!prefilled ?
          this.fillSelectedWithSolution : null}
        >
          <LoupeIcon />Hint
        </div>
        <style jsx>{ActionsStyle}</style>
      </div>
    );
  }
...
}