Leaflet implementation example

leaflet.js

import css from "../css/app.css"
import '../css/leaflet.css';

import * as L from './leaflet-src.esm.js';

import axios from 'axios';

window.addEventListener('load', () => {
  const maxTheMap = () => {
    let map = document.getElementById("map");
    let header_height = document.getElementById("site-header").clientHeight;

    // map.style.width = "";
    map.style.height = (window.innerHeight - header_height) + "px";
  }
  maxTheMap();

  const map = L.map('map', {
    preferCanvas: true,
    center: [43.0389,-87.90647],
    zoom: 13
  });

  L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
    maxZoom: 18,
    id: 'elouisyoung.0f89goo2',
    accessToken: 'pk.eyJ1IjoiZWxvdWlzeW91bmciLCJhIjoiY2lwcGxlaWVpMDQzb2Z3bmN1dmhuMXQxMyJ9.YcVQVtBpfDfKI2KXfil6NA'
  }).addTo(map);

  const markerLayer = L.layerGroup();
  markerLayer.addTo(map);

  const updateMarkers = (map) => {
    axios({
      method: 'get',
      url: '/api/markers/in_bounds',
      params: {
        nw_lat: map.getBounds().getNorthWest().lat,
        nw_lng: map.getBounds().getNorthWest().lng,
        se_lat: map.getBounds().getSouthEast().lat,
        se_lng: map.getBounds().getSouthEast().lng
      }
    })
    .then( response => {

      // remove out of bounds markers
      markerLayer.getLayers().forEach( m => {
        if ( m.getLatLng().lat > map.getBounds().getNorthWest().lat
          || m.getLatLng().lat < map.getBounds().getSouthEast().lat
          || m.getLatLng().lng < map.getBounds().getNorthWest().lng
          || m.getLatLng().lng > map.getBounds().getSouthEast().lng
        ) {
          markerLayer.removeLayer(m);
        }
      });

      // iterate over new markers for current bounds
      // for each new marker, iterate over all existing
      // add if not already in markerLayer, do nothing if found
      response.data.markers.forEach( responseMarker => {

        // iterate over each existing Markers
        // only add if not found in markerLayer
        let alreadyFound = markerLayer.getLayers().reduce( (bool_acc, existingMarker) => {
          return bool_acc || ( responseMarker.id == existingMarker.options.id );
        }, false);
        if ( !alreadyFound ) {
          L.marker([responseMarker.lat, responseMarker.lng], {id: responseMarker.id}).addTo(markerLayer);
        }
      });

    });
  }

  const moveEnd = (e) => {
    updateMarkers(e.sourceTarget);
  }

  map.addEventListener('moveend', moveEnd);
  updateMarkers(map);

  window.addEventListener('resize', maxTheMap);
})

the main thing I wanted to share here was the handling of duplicate map markers returned by the api.
the api endpoint is returning all markers within the given map bounds every time the map stops at a new position.
we only want to render a marker on the map one time...
basically I maintain a "layerGroup" (line 30) and add or remove markers to that group intelligently