OL-Cesium Uncaught (in promise) SyntaxError

Question

Trying to implement OL-Cesium as an ES6 library following instructions here: https://openlayers.org/ol-cesium/apidoc/#as-an-es6-library-recommended-method

Running locally for now and getting the following error:

Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0 at JSON.parse ()

The error is being thrown when attempting to fetch "Assets/approximateTerrainHeights.json". The response is HTML (the index page, in fact) and not JSON, hence the error. But for the life of me, I can't understand why. The header shows the request URL as correct (http://localhost:1234/static/Assets/approximateTerrainHeights.json) with a Status Code of 200, but the response is the index.html page.

Full js code for reference:

    import '/css/index.css';
import 'ol/ol.css';

import {Map, View} from "ol";
import TileLayer from 'ol/layer/Tile';
import XYZ from 'ol/source/XYZ.js';
import * as olProj from 'ol/proj.js';


window.CESIUM_BASE_URL = '/static/';


import * as Cesium from 'cesium';
window.Cesium = Cesium; // expose Cesium to the OL-Cesium library
require('cesium/Build/Cesium/Widgets/widgets.css');

import OLCesium from 'olcs/OLCesium.js';
import 'olcs/olcs.css';


let tileWorldImagery = new TileLayer({
    source: new XYZ({
      url: 'http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}',
      crossOrigin: 'Anonymous',
    })
  });

let map = new Map({
  target: "map",
  projection: 'EPSG:3857',
  layers: [
    tileWorldImagery
  ],
  view: new View({
    center: olProj.fromLonLat([134.364805, -26.710497]),
    zoom: 4,
    minZoom: 2,
  }),
});

const ol3d = new OLCesium({map: map}); // map is the ol.Map instance
ol3d.setEnabled(true);
window.ol3d = ol3d; // temporary hack for easy console debugging

Answer 1

Turns out that window.CESIUM_BASE_URL is configured correctly, but I had erroneously placed the 'static' directory referenced in the CESIUM_BASE_URL in the wrong location. I originally had the 'static' directory placed in the project root, but once I dropped it into the 'dist' directory where the app is compiled to, everything works like a charm.

Hope this saves someone else a few headaches. Thanks to Mike and nmtoken for setting me down the right path.