three.js/examples/jsm/nodes/lighting/LightUtils.js

import { tslFn } from '../shadernode/ShaderNode.js';

export const getDistanceAttenuation = tslFn( ( inputs ) => {

	const { lightDistance, cutoffDistance, decayExponent } = inputs;

	// based upon Frostbite 3 Moving to Physically-based Rendering
	// page 32, equation 26: E[window1]
	// https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
	const distanceFalloff = lightDistance.pow( decayExponent ).max( 0.01 ).reciprocal();

	return cutoffDistance.greaterThan( 0 ).cond(
		distanceFalloff.mul( lightDistance.div( cutoffDistance ).pow4().oneMinus().clamp().pow2() ),
		distanceFalloff
	);

} ); // validated