Error : Cannot read property 'map' of undefined

Question

I'm following the reactjs tutorial, and I keep running into an issue when
passing the value from the state of one component into another component.

The error Cannot read property 'map' of undefined' is thrown when the map function in the CommentList component is executed.

What would cause the prop to be undefined when passing from CommentBox into the CommentList?

// First component var CommentList = React.createClass({   render: function() {     var commentNodes = this.props.data.map(function (comment){       return <div><h1>{comment.author}</h1></div>;     });     return <div className="commentList">{commentNodes}</div>;   } }); // Second component     var CommentBox = React.createClass({    getInitialState: function(){      return {data: []};    },    getComments: function(){       $.ajax({         url: this.props.url,         dataType: 'json',         success: function(data){ this.setState({data: data}) }.bind(this),         error: function(xhr, status, err){ console.error(url, status, err.toString()) }.bind(this)       });    },    componentWillMount: function(){       this.getComments()    },    render: function(){       return <div className="commentBox">{<CommentList data={this.state.data.comments}/>}</div>;    } });  React.renderComponent( <CommentBox url="comments.json" />, document.getElementById('content')); 

Answer 1

First of all, set more safe initial data:

getInitialState : function() {     return {data: {comments:[]}}; }, 

And ensure your ajax data.

It should work if you follow above two instructions like Demo.

Updated: you can just wrap the .map block with conditional statement.

if (this.props.data) {   var commentNodes = this.props.data.map(function (comment){       return (         <div>           <h1>{comment.author}</h1>         </div>       );   }); }