Communicate Between Components

For parent-child communication, simply pass props.

For child-parent communication: Say your GroceryList component has a list of items generated through an array. When a list item is clicked, you want to display its name:

var GroceryList = React.createClass({
  handleClick: function(i) {
    console.log('You clicked: ' + this.props.items[i]);
  },

  render: function() {
    return (
      <div>
        {this.props.items.map(function(item, i) {
          return (
            <div onClick={this.handleClick.bind(this, i)} key={i}>{item}</div>
          );
        }, this)}
      </div>
    );
  }
});

React.render(
  <GroceryList items={['Apple', 'Banana', 'Cranberry']} />, mountNode
);

Notice the use of bind(this, arg1, arg2, ...): we're simply passing more arguments to handleClick. This is not a new React concept; it's just JavaScript.

For communication between two components that don't have a parent-child relationship, you can set up your own global event system. Subscribe to events in componentDidMount(), unsubscribe in componentWillUnmount(), and when you receive an event, call setState().