JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/ol3/3.18.2/ol.js"></script>

JavaScript

var source = 

/* Valid geometries:
  2.1.2. Point
  2.1.3. MultiPoint
  2.1.4. LineString
  2.1.5. MultiLineString
  2.1.6. Polygon
  2.1.7. MultiPolygon

/* Use OpenLayers 3 for parsing GeoJSON data */

*/
source.type = "MultiPolygon";
var geoJSONFormat = new ol.format.GeoJSON();
var geoJSONGeometry = geoJSONFormat.readGeometry(source).transform("EPSG:3857", "EPSG:4326");
var geoJSONFeature = new ol.Feature(geoJSONGeometry);

/* Open Browser console and see result */
console.log(geoJSONFormat.writeFeature(geoJSONFeature));