[javascript] React JS - Uncaught TypeError: this.props.data.map is not a function

I'm working with reactjs and cannot seem to prevent this error when trying to display JSON data (either from file or server):

Uncaught TypeError: this.props.data.map is not a function

I've looked at:

React code throwing “TypeError: this.props.data.map is not a function”

React.js this.props.data.map() is not a function

Neither of these has helped me fix the problem. After my page loads, I can verify that this.data.props is not undefined (and does have a value equivalent to the JSON object - can call with window.foo), so it seems like it isn't loading in time when it is called by ConversationList. How do I make sure that the map method is working on the JSON data and not an undefined variable?

var converter = new Showdown.converter();

var Conversation = React.createClass({
  render: function() {
    var rawMarkup = converter.makeHtml(this.props.children.toString());
    return (
      <div className="conversation panel panel-default">
        <div className="panel-heading">
          <h3 className="panel-title">
            {this.props.id}
            {this.props.last_message_snippet}
            {this.props.other_user_id}
          </h3>
        </div>
        <div className="panel-body">
          <span dangerouslySetInnerHTML={{__html: rawMarkup}} />
        </div>
      </div>
    );
  }
});

var ConversationList = React.createClass({
  render: function() {

    window.foo            = this.props.data;
    var conversationNodes = this.props.data.map(function(conversation, index) {

      return (
        <Conversation id={conversation.id} key={index}>
          last_message_snippet={conversation.last_message_snippet}
          other_user_id={conversation.other_user_id}
        </Conversation>
      );
    });

    return (
      <div className="conversationList">
        {conversationNodes}
      </div>
    );
  }
});

var ConversationBox = React.createClass({
  loadConversationsFromServer: function() {
    return $.ajax({
      url: this.props.url,
      dataType: 'json',
      success: function(data) {
        this.setState({data: data});
      }.bind(this),
      error: function(xhr, status, err) {
        console.error(this.props.url, status, err.toString());
      }.bind(this)
    });
  },
  getInitialState: function() {
    return {data: []};
  },
  componentDidMount: function() {
    this.loadConversationsFromServer();
    setInterval(this.loadConversationsFromServer, this.props.pollInterval);
  },
  render: function() {
    return (
      <div className="conversationBox">
        <h1>Conversations</h1>
        <ConversationList data={this.state.data} />
      </div>
    );
  }
});

$(document).on("page:change", function() {
  var $content = $("#content");
  if ($content.length > 0) {
    React.render(
      <ConversationBox url="/conversations.json" pollInterval={20000} />,
      document.getElementById('content')
    );
  }
})

EDIT: adding sample conversations.json

Note - calling this.props.data.conversations also returns an error:

var conversationNodes = this.props.data.conversations.map...

returns the following error:

Uncaught TypeError: Cannot read property 'map' of undefined

Here is conversations.json:

{"user_has_unread_messages":false,"unread_messages_count":0,"conversations":[{"id":18768,"last_message_snippet":"Lorem ipsum","other_user_id":10193}]}

I had the same problem. The solution was to change the useState initial state value from string to array. In App.js, previous useState was

const [favoriteFilms, setFavoriteFilms] = useState('');

I changed it to

const [favoriteFilms, setFavoriteFilms] = useState([]);

and the component that uses those values stopped throwing error with .map function.

It happens because the component is rendered before the async data arrived, you should control before to render.

I resolved it in this way:

render() {
    let partners = this.props && this.props.partners.length > 0 ?
        this.props.partners.map(p=>
            <li className = "partners" key={p.id}>
                <img src={p.img} alt={p.name}/> {p.name} </li>
        ) : <span></span>;

    return (
        <div>
            <ul>{partners}</ul>
        </div>
    );
}

• Map can not resolve when the property is null/undefined, so I did a control first

this.props && this.props.partners.length > 0 ?

If you're using react hooks you have to make sure that data was initialized as an array. Here's is how it must look like:

const[data, setData] = useState([])

You don't need an array to do it.

var ItemNode = this.state.data.map(function(itemData) {
return (
   <ComponentName title={itemData.title} key={itemData.id} number={itemData.id}/>
 );
});

try componentDidMount() lifecycle when fetching data

You need to convert the object into an array to use the map function:

const mad = Object.values(this.props.location.state);

where this.props.location.state is the passed object into another component.

More generally, you can also convert the new data into an array and use something like concat:

var newData = this.state.data.concat([data]);  
this.setState({data: newData})

This pattern is actually used in Facebook's ToDo demo app (see the section "An Application") at https://facebook.github.io/react/.

As mentioned in the accepted answer, this error is usually caused when the API returns data in a format, say object, instead of in an array.

If no existing answer here cuts it for you, you might want to convert the data you are dealing with into an array with something like:

let madeArr = Object.entries(initialApiResponse)

The resulting madeArr with will be an array of arrays.

This works fine for me whenever I encounter this error.

what worked for me is converting the props.data to an array using data = Array.from(props.data); then I could use the data.map() function

I had a similar error, but I was using Redux for state management.

My Error:

Uncaught TypeError: this.props.user.map is not a function

What Fixed My Error:

I wrapped my response data in an array. Therefore, I can then map through the array. Below is my solution.

const ProfileAction = () => dispatch => {
  dispatch({type: STARTFETCHING})
  AxiosWithAuth()
    .get(`http://localhost:3333/api/users/${id here}`)
    .then((res) => {
        // wrapping res.data in an array like below is what solved the error 
        dispatch({type: FETCHEDPROFILE, payload: [res.data]})
    }) .catch((error) => {
        dispatch({type: FAILDFETCH, error: error})
    })
 }

 export default ProfileAction