App.js

  addDonor = (name, mobile, group) => {
    if(this.state.name != null && this.state.mobile != null && this.state.group != null){ 
      fetch('https://blood-donors-db.firebaseio.com/donors.json', {
        method: 'POST',
        headers: {
          Accept: 'application/json',
          'Content-Type': 'application/json',
        },
        body: JSON.stringify({
          "name": name,
          "mobile": mobile,
          "group": group,
        }),
      })
      .then((response) => response.json())
      .then((responseData) => {
              if(responseData.name != null ){
                this.setState({
                    name: null,
                    mobile: null,
                    group: null,
                    isSubmited: true,
                  })              
              }
              else{
              Alert.alert(
                'Oops !',
                'Something went wrong',
                [
                  {text: 'OK', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},
                ],
                { cancelable: false }
              )
            }

      })
      .done();
    }
      else{
        Alert.alert(
          'Oops !',
          'You forgot some field. Please fill it before submitting',
          [
            {text: 'OK', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},
          ],
          { cancelable: false }
        )        
      }
    
  }