"Uncaught SyntaxError: Cannot use import statement outside a module" when importing ECMAScript 6

I'm using ArcGIS JSAPI 4.12 and wish to use Spatial Illusions to draw military symbols on a map.

When I add milsymbol.js to the script, the console returns error

Uncaught SyntaxError: Cannot use import statement outside a module`

so I add type="module" to the script, and then it returns

Uncaught ReferenceError: ms is not defined

Here's my code:

<link rel="stylesheet" href="https://js.arcgis.com/4.12/esri/css/main.css">
<script src="https://js.arcgis.com/4.12/"></script>
<script type="module" src="milsymbol-2.0.0/src/milsymbol.js"></script>

<script>
    require([
        "esri/Map",
        "esri/views/MapView",
        "esri/layers/MapImageLayer",
        "esri/layers/FeatureLayer"
    ], function (Map, MapView, MapImageLayer, FeatureLayer) {

        var symbol = new ms.Symbol("SFG-UCI----D", { size: 30 }).asCanvas(3);
        var map = new Map({
            basemap: "topo-vector"
        });

        var view = new MapView({
            container: "viewDiv",
            map: map,
            center: [121, 23],
            zoom: 7
        });
    });
</script>

So, whether I add type="module" or not, there are always errors. However, in the official document of Spatial Illusions, there isn't any type="module" in the script. I'm now really confused. How do they manage to get it work without adding the type?

File milsymbol.js

import { ms } from "./ms.js";

import Symbol from "./ms/symbol.js";
ms.Symbol = Symbol;

export { ms };

Update For Node.js / NPM

Add "type": "module" to your package.json file.

{
  // ...
  "type": "module",
  // ...
}

Note: When using modules, if you get ReferenceError: require is not defined, you'll need to use the import syntax instead of require. You can't natively mix and match between them, so you'll need to pick one or use a bundler if you need to use both.