ensure that we do not index into a string

[gavinr-maps]

Addresses #1416

Fixing an issue from #1411 where in some cases we could be indexing into a string.

Test case:

Click to expand

<script src="https://unpkg.com/proj4@2.4.3"></script>
  <script src="https://unpkg.com/proj4leaflet@1.0.1"></script>
  <div id="map"></div>
  <script>
    /* create new Proj4Leaflet CRS:
      1. Proj4 and WKT definitions can be found at sites like https://epsg.io, https://spatialreference.org/ or by using gdalsrsinfo https://www.gdal.org/gdalsrsinfo.html
      2. Appropriate values to supply to the resolution and origin constructor options can be found in the ArcGIS Server tile server REST endpoint (ex: https://tiles.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer).
      3. The numeric code within the first parameter (ex: `27700`) will be used to project the dynamic map layer on the fly
    */
    const crs = new L.Proj.CRS(
      "EPSG:27700",
      "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs",
      {
        origin: [-5781523.997920001, 4883853.592504997],
        resolutions: [
          132291.9312505292,
          66145.9656252646,
          26458.386250105836,
          19843.789687579378,
          13229.193125052918,
          6614.596562526459,
          2645.8386250105837,
          1322.9193125052918,
          661.4596562526459,
          264.5838625010584,
          132.2919312505292,
          66.1459656252646,
          26.458386250105836,
          19.843789687579378,
          13.229193125052918,
          6.614596562526459,
          2.6458386250105836,
          1.3229193125052918,
          0.6614596562526459
        ]
      }
    );

    const map = L.map("map", {
      crs: crs
    }).setView([53.386, -2.319], 1);

    // The min/maxZoom values provided should match the actual cache thats been published. This information can be retrieved from the service endpoint directly.
    L.esri
      .tiledMapLayer({
        url: "https://tiles.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer",
        maxZoom: 19,
        minZoom: 1,
        attribution:  "Powered by <a href='https://www.esri.com/en-us/home' target='_blank'>Esri</a>"
      })
      .addTo(map);
  </script>