Q:

React Leaflet first object is undefined in a useState case for drawing map markers

I am trying to loop through an object that has been parsed from a CSV file of Locations, and Lat/Lng pairs. But when I loop through this object, the first loop returns my object with all the values as undefined. My code is as follows: 
import React, { Component, useState, useEffect } from 'react';
import { LatLng } from 'leaflet';
import { Map, TileLayer, CircleMarker, Popup, Marker } from 'react-leaflet';
import CovidCasesClean from './CovidCasesClean.csv';
import Papa from 'papaparse';

export default function PlaceMarker() {
    const latlng = [null];
    const [rows, setRows] = React.useState([])
    const [data, setData] = React.useState({});

    React.useEffect(() => {
      async function getData() {
        const response = await fetch('/data/CovidCasesClean.csv')
        const reader = response.body.getReader()
        const result = await reader.read() // raw array
        const decoder = new TextDecoder('utf-8')
        const csv = decoder.decode(result.value) // the csv text
        console.log(csv)
        const results = Papa.parse(csv, { header: true, skipEmptyLines: true}) // object with { data, errors, meta }
        console.log(results)
        const rows = results.data
        console.log(rows)
        // array of objects
        setRows(rows)
        rows.map((item, i) => {
          const rowObj = {item}
          //console.log(rowObj)
          console.log(i)
          setData({
            ...data, key: i, ...data, location: rowObj.item.Location, ...data, latitude: parseFloat(rowObj.item.Latitude), ...data, longitude: parseFloat(rowObj.item.Longitude), ...data, cases: parseFloat(rowObj.item.TotalCases)
          })
        })
      }
      getData()
    }, []) // [] means just do this once, after initial render
    return (
      <div>{console.log(data.key, data.location, data.latitude, data.longitude, data.cases)}</div> 
    )
  }

When the console line: <div>{console.log(data.key, data.location, data.latitude, data.longitude, data.cases)}</div> is ran, the first result returns: 

undefined undefined undefined undefined undefined

whereas the second run through returns the correct object like so: 

0 Barking and Dagenham 53.546299 -1.46786 18 

This causes my creating a marker object to fail as the first object is not a latlng pair, I have been trying to create markers like so:
<Marker position={(data.longitude, data.latitude)}>
  <Popup>
    <span>
      A pretty CSS3 popup.
      <br />
      Easily customizable.
    </span>
  </Popup>
</Marker>;

This is where the error occurs ^ 
Any ideas where I am going wrong, this is my first react project any help appreciated!
CSV file format: 
Location,TotalCases,Latitude,Longitude
Barking and Dagenham,18,53.546299,-1.46786
Barnet,28,51.605499,-0.207715

A:

What you are trying can be achieved in a much simpler way.
Parse the csv data inside an useEffectHook. You do not need any of these:

const response = await fetch('/data/CovidCasesClean.csv')
      const reader = response.body.getReader()
      const result = await reader.read() // raw array
      const decoder = new TextDecoder('utf-8')
      const csv = decoder.decode(result.value) // the csv text
      console.log(csv)

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

  useEffect(() => {
    Papa.parse(csv, {
        download: true, // use this option to interpret the input string as a URL from which to download the input file.
        header: true,
        skipEmptyLines: true,
        complete: results => setData(results.data)
    });
  }, []);

and once you store the csv parsed markers data loop over them to visualize the markers:
{data &&
    data.map(({ Location, Latitude, Longitude, TotalCases }, i) => (
      <Marker
        key={`markers-${i}`}
        position={[Latitude, Longitude]}
        icon={icon}
      >
        <Popup>
          <span>
            <b>Location</b>: {Location}
            <br />
            <b>TotalCases</b>: {TotalCases}
          </span>
        </Popup>
      </Marker>
    ))}

Demo

