Three.jsのSTLExporterでSTL出力をしたいが、「コンストラクタでない」とエラーが出る

前提・実現したいこと

Three.jsを使い、コードで記述した立方体をSTLで出力させたい。
Three.jsのexsampleに入っているSTLExporterを使用していますが、うまく動きません。

発生している問題・エラーメッセージ

以下のエラーが出て、画面にボタン以外が表示されず真っ白になる。

Uncaught TypeError: THREE.STLExporter is not a constructor
    at init (test02.html:49)

該当のソースコード

<!DOCTYPE HTML>

<html lang="ja">

<head>
    <meta charset="utf-8">
    <title>test 2</title>
    <script type="text/javascript" src="./libs/three.js"></script>
    

    <script type="text/javascript" src="./libs/stats.min.js"></script>
    <script type="text/javascript" src="./libs/dat.gui.min.js"></script>
    
    <style>
        body {
            /* set margin to 0 and overflow to hidden, to go fullscreen */
            margin: 0;
            overflow:visible;
        }
    </style>
</head>
<body>

<div id="Stats-output">
</div>
<!-- Div which will hold the Output -->
<div id="WebGL-output">
</div>
    
    <button id="export">STLで出力</button>

<!-- Javascript code that runs our Three.js examples -->
<script type="module">
    import * as THREE from './libs/three.module.js';
    import * as STLExporter from './libs/exporters/STLExporter.js';


    
    function init() {

        var stats = initStats();

        //シーン作成
        var scene = new THREE.Scene();
        var mesh;
        var exporter;
        
        exporter = new THREE.STLExporter();
        
        
        //カメラ
        var camera = new THREE.PerspectiveCamera(45, window.innerWidth / window.innerHeight, 0.1, 1000);

        //renderer
        var webGLRenderer = new THREE.WebGLRenderer();
        webGLRenderer.setClearColor(new THREE.Color(0xaaaaff));
        webGLRenderer.setSize(window.innerWidth, window.innerHeight);
        webGLRenderer.shadowMap.enabled = true;
        
        //planeの作成
        var planeGeometry = new THREE.PlaneGeometry(60, 20);
        var planeMaterial = new THREE.MeshBasicMaterial({color: 0xcccccc});
        var plane = new THREE.Mesh(planeGeometry, planeMaterial);

        plane.rotation.x = -0.5 * Math.PI;
        plane.position.x = 0;
        plane.position.y = 0;
        plane.position.z = 0;

        //planeをシーンに追加
        scene.add(plane);
        
        


        //カメラの位置
        camera.position.x = 0;
        camera.position.y = 40;
        camera.position.z = 50;
        camera.lookAt(new THREE.Vector3(0, 10, 0));


        //ライト
        var spotLight = new THREE.SpotLight(0xffffff);
        spotLight.position.set(0, 40, 30);
        spotLight.intensity = 2;
        scene.add(spotLight);

        //レンダラー
        document.getElementById("WebGL-output").appendChild(webGLRenderer.domElement);

        
        var step = 0;
        
        //グリッド線の表示
        var grid = new THREE.GridHelper( 2000, 20, 0x000000, 0x000000 );
            grid.material.opacity = 0.2;
            grid.material.transparent = true;
            scene.add( grid );

        //control
        var controls = new function () {
            // we need the first child, since it's a multimaterial
        
        //export STLの部分
        var buttonExportASCII = document.getElementById( 'export' );
        buttonExportASCII.addEventListener( 'click', exportASCII );


        };
        
        var geometry = new THREE.BoxBufferGeometry( 5, 5, 5 );
            var material = new THREE.MeshPhongMaterial( { color: 0x00ff00 } );

            mesh = new THREE.Mesh( geometry, material );
            mesh.castShadow = true;
            mesh.position.y = 10;
            mesh.position.z = 5;
            scene.add( mesh );


        var gui = new dat.GUI();
        
        //gui.addColor(geometry, 'fillColor');//塗りの色。カラーピッカーを出す,


        render();


        function render() {
            stats.update();

            


            // render using requestAnimationFrame
            requestAnimationFrame(render);
            webGLRenderer.render(scene, camera);
        }
        
        
        
        function exportASCII() {
                var result = exporter.parse( mesh, { binary: true } );
				saveString( result, 'box.stl' );

			}

			

			var link = document.createElement( 'a' );
			link.style.display = 'none';
			document.body.appendChild( link );

			function save( blob, filename ) {

				link.href = URL.createObjectURL( blob );
				link.download = filename;
				link.click();

			}

			function saveString( text, filename ) {

				save( new Blob( [ text ], { type: 'text/plain' } ), filename );

			}

			function saveArrayBuffer( buffer, filename ) {

				save( new Blob( [ buffer ], { type: 'application/octet-stream' } ), filename );

			}

        function initStats() {

            var stats = new Stats();
            stats.setMode(0); // 0: fps, 1: ms


            stats.domElement.style.position = 'absolute';
            stats.domElement.style.left = '0px';
            stats.domElement.style.top = '0px';

            document.getElementById("Stats-output").appendChild(stats.domElement);

            return stats;
        }
    }
    window.onload = init;
</script>
</body>
</html>