React.createElement: type should not be null or undefined. It should be a string (for DOM elements) or a ReactClass (for composite components).

(React.js ver.0.13)

var ListView = React.createClass({
    render: function() {
        var items = this.props.data.map(function(item) {
            return ItemDelegate({key:item.id,data:item.id});
        }.bind(this));
        return (React.createElement('ul',null,items));
    }
});

var ItemDelegate = React.createFactory(ListViewChildren);

var ListViewChildren = React.createClass({
    render: function(){
        return (React.createElement('li',{key: this.props.key,data: this.props.data}));
    }
});

var Wrapper = React.createClass({
    render: function() {
        return (React.createElement(ListView,{data: [/*some data*/]}));
    }
});

React.render(React.createElement(Wrapper),document.body);

它是什么类型，在哪里声明类型必须被声明？我想这个概念有一个完全的误会，但在哪里？