Trying load gltf model, but see nothing (lights are added, camera is set) (blender) (react + three)

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I’ve polished your code a bit so you have proper camera and light settings. Besides, the animation of the sphere should correctly play:

import React, { Component } from 'react'
import ReactDOM from 'react-dom'
import './index.css'
import * as THREE from 'three'
import { GLTFLoader } from 'three/examples/jsm/loaders/GLTFLoader'
import { OrbitControls } from 'three/examples/jsm/controls/OrbitControls'

class App extends Component {
	componentDidMount() {
		// === THREE.JS CODE START ===

		var mixer;

		var gltfLoader = new GLTFLoader();
		var scene = new THREE.Scene();
		scene.background = new THREE.Color( 0x808080 );

		var camera = new THREE.PerspectiveCamera( 45, window.innerWidth / window.innerHeight, 1, 1000 );
		camera.position.set( 50, 50, 50 );

		var renderer = new THREE.WebGLRenderer();
		renderer.setPixelRatio( window.devicePixelRatio );
		renderer.setSize( window.innerWidth, window.innerHeight );
		this.mount.appendChild( renderer.domElement );

		var hemiLight = new THREE.HemisphereLight( 0xffffff, 0x444444 );
		hemiLight.position.set( 0, 20, 0 );
		scene.add( hemiLight );

		var dirLight = new THREE.DirectionalLight( 0xffffff );
		dirLight.position.set( - 3, 10, - 10 );
		scene.add( dirLight );


		gltfLoader.load( './isometric-ruletik-test.glb', gltf => {
			console.log(gltf);
			scene.add( gltf.scene );

			mixer = new THREE.AnimationMixer( gltf.scene );
			mixer.clipAction( gltf.animations[ 0 ] ).play();

		} );


		const helper = new THREE.CameraHelper( camera );
		scene.add( helper );


		const controls = new OrbitControls( camera, renderer.domElement );

		const clock = new THREE.Clock();

		function animate() {

			requestAnimationFrame( animate );

			var delta = clock.getDelta();

			if ( mixer ) mixer.update( delta );

			renderer.render(scene,camera);

		}

		animate();

		// === THREE.JS EXAMPLE CODE END ===

	}

	render() {
		return (
			<div ref={ref => (this.mount = ref)} />
		)
	}
}

export default App

ReactDOM.render(<App />, document.getElementById('root'))

The most important bit is the usage of an animation loop. Besides, you get better visuals by using an perspective camera.