three.js/examples/jsm/nodes/functions/light/getDirectionVector.js

import { Vector3 } from 'three';

let vector3;

const getDirectionVector = ( light, camera, directionVector ) => {

	vector3 || ( vector3 = new Vector3() );

	directionVector.setFromMatrixPosition( light.matrixWorld );
	vector3.setFromMatrixPosition( light.target.matrixWorld );
	directionVector.sub( vector3 );
	directionVector.transformDirection( camera.matrixWorldInverse );

};

export default getDirectionVector;