React Native Possible unhandled promise rejection

Question

I m getting the following error  Possible unhandled promise rejection  id 0  Network request failed   Here s the promise code  I don t see what s wrong here  any ideas       return fetch url       then function response         return response json                then function json         return           city  json name          temperature  kelvinToF json main temp           description    capitalize json weather 0  description                      catch function error        console log  There has been a problem with your fetch operation      error message                 Edit  I added a catch function and got a better error You passed an undefined or null state object  instead  use forceUpdate    index ios js 64 undefined  Here s the index ios js code  The url is fine and giving me the correct json data  I can see with console log that both region latitude and region longitude are available in Api region latitude  region longitude   But data is undefined  I m still not sure what s going on  why there s a problem with data and why it s undefined      var React   require  react-native    --deprecated     updated import React from  react       updated import     AppRegistry    MapView    View    Text    StyleSheet    from  react-native       var     AppRegistry    MapView    View    Text    StyleSheet     React        -- depreciated    var Api   require    src api     var Weather   React createClass     getInitialState  function         return         pin            latitude  0          longitude  0                city            temperature            description                   render  function         return  lt View style  styles container  gt         lt MapView         annotations   this state pin           onRegionChangeComplete  this onRegionChangeComplete          style  styles map  gt         lt  MapView gt         lt View style  styles textWrapper  gt           lt Text style  styles text  gt  this state city  lt  Text gt           lt Text style  styles text  gt  this state temperature  lt  Text gt           lt Text style  styles text  gt  this state description  lt  Text gt         lt  View gt       lt  View gt         onRegionChangeComplete  function region        this setState         pin            longitude  region longitude          latitude  region latitude                      Api region latitude  region longitude         then  data    gt            console log data           this setState data                         var styles   StyleSheet create     container        flex  1      justifyContent   center       alignItems   stretch       backgroundColor    F5FCFF         map        flex  2      marginTop  30        textWrapper        flex  1      alignItems   center         text        fontSize  30          AppRegistry registerComponent  weather        gt  Weather

User · Answer

In My case  I am running a local Django backend in IP 127 0 0 1 8000 with Expo start  Just make sure you have the server in public domain not hosted lcoally on your machine

User · Answer

You should add the catch   to the end of the Api call  When your code hits the catch   it doesn t return anything  so data is undefined when you try to use setState   on it  The error message actually tells you this too

User · Answer

delete build folder projectfile android app build and run project

User · Answer

catch function in your api should either return some data which could be handled by Api call in React class or throw new error which should be caught using a catch function in your React class code  Latter approach should be something like     return fetch url   then function response     return response json        then function json     return       city  json name      temperature  kelvinToF json main temp       description    capitalize json weather 0  description          catch function error    console log  There has been a problem with your fetch operation      error message       ADD THIS THROW error   throw error        Then in your React Class   Api region latitude  region longitude     then  data    gt        console log data       this setState data        catch  error   gt        console log  Api call error         alert error message

User · Answer

Adding here my experience that hopefully might help somebody    I was experiencing the same issue on Android emulator in Linux with hot reload  The code was correct as per accepted answer and the emulator could reach the internet  I needed a domain name    Refreshing manually the app made it work  So maybe it has something to do with the hot reloading

User · Answer

According to this post  you should enable it in XCode    Click on your project in the Project Navigator Open the Info tab Click on the down arrow left to the  App Transport Security Settings  Right click on  App Transport Security Settings  and select Add Row For created row set the key    Allow Arbitrary Loads     type to boolean and value to YES

[react-native] React Native: Possible unhandled promise rejection

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