three.js/examples/jsm/helpers/VertexNormalsHelper.js

import {
	BufferGeometry,
	Float32BufferAttribute,
	LineSegments,
	LineBasicMaterial,
	Matrix3,
	Vector3
} from 'three';

const _v1 = new Vector3();
const _v2 = new Vector3();
const _normalMatrix = new Matrix3();

class VertexNormalsHelper extends LineSegments {

	constructor( object, size = 1, color = 0xff0000 ) {

		const geometry = new BufferGeometry();

		const nNormals = object.geometry.attributes.normal.count;
		const positions = new Float32BufferAttribute( nNormals * 2 * 3, 3 );

		geometry.setAttribute( 'position', positions );

		super( geometry, new LineBasicMaterial( { color, toneMapped: false } ) );

		this.object = object;
		this.size = size;
		this.type = 'VertexNormalsHelper';

		//

		this.matrixAutoUpdate = false;

		this.update();

	}

	update() {

		this.object.updateMatrixWorld( true );

		_normalMatrix.getNormalMatrix( this.object.matrixWorld );

		const matrixWorld = this.object.matrixWorld;

		const position = this.geometry.attributes.position;

		//

		const objGeometry = this.object.geometry;

		if ( objGeometry ) {

			const objPos = objGeometry.attributes.position;

			const objNorm = objGeometry.attributes.normal;

			let idx = 0;

			// for simplicity, ignore index and drawcalls, and render every normal

			for ( let j = 0, jl = objPos.count; j < jl; j ++ ) {

				_v1.fromBufferAttribute( objPos, j ).applyMatrix4( matrixWorld );

				_v2.fromBufferAttribute( objNorm, j );

				_v2.applyMatrix3( _normalMatrix ).normalize().multiplyScalar( this.size ).add( _v1 );

				position.setXYZ( idx, _v1.x, _v1.y, _v1.z );

				idx = idx + 1;

				position.setXYZ( idx, _v2.x, _v2.y, _v2.z );

				idx = idx + 1;

			}

		}

		position.needsUpdate = true;

	}

	dispose() {

		this.geometry.dispose();
		this.material.dispose();

	}

}

export { VertexNormalsHelper };