title: Using the map function in React Native
body: <p>I am trying to render markers on a MapView for each position stored inside my state array called markers, but for some reason when I try to reference the state inside the map function I get undefined. I checked for syntax errors but couldn't find the problem. Basically every time a user changes his/her location I want to set their marker to their new location. The error occurs when trying to use the map function inside the render() function. I have posted a picture of the error below.</p>
  
  <pre><code>import React, {Component} from 'react';
  import {AppRegistry,Text,View,StyleSheet} from 'react-native'
  import Component1 from`enter code here` './Component1'
  import Component3 from './Component3'
  import Component4 from './Component4'
  import MapView from 'react-native-maps'
  
  export default class myapp extends Component{
    constructor(props){
      super();
      this.state ={
        markers: [
          {
            watchId: 0,
            latitude:3,
            longitude:2
          },
          {
            watchId: 2,
            latitude:3,
            longitude:2
          }
        ]
  
      }
    }
  
    componentDidMount(){
      navigator.geolocation.watchPosition((position)=&gt;{
        var lat = parseFloat(position.coords.latitude)
        var long = parseFloat(position.coords.longitude)
        var id = parseFloat(position.coords.watchId)
        var found = false
        this.setState({markers: this.state.markers.map(marker =&gt;{
          if(marker.watchId === id){
            found = true
            marker.latitude = lat
            marker.longitude = long
          }
        })});
      if(found === false){
        this.setState({markers:[{watchId:id,latitude:lat,longitude:long}]});
      }
      },null,{timeout:0,distanceFilter:0})
  }
    render(){
      return(
        &lt;View&gt;
        &lt;MapView style={{height:350,width:350,marginTop:300,alignSelf:'center'}}
        initialRegion={{
          latitude: 37.33,
          longitude: -121.4024,
          latitudeDelta: 0.0922,
          longitudeDelta: 0.0421,
        }}
        showsUserLocation={true}
        followsUserLocation={true}
        showsMyLocationButton={true}
        &gt;
        {this.state.markers.map(marker =&gt;{
          return(
                  &lt;MapView.Marker
                  coordinate={{latitude:marker.latitude,longitude:marker.longitude}}
                  title={"title"}
                  description={"description ey"}
  
               /&gt;
          );
        })}
  
        &lt;/MapView&gt;
        &lt;/View&gt;
  
      );
    }
  }
  AppRegistry.registerComponent('myapp',()=&gt;myapp);
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/SvNee.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SvNee.png" alt="The Error"></a></p>