three.js/src/renderers/shaders/ShaderChunk/specularmap_fragment.glsl.js

export default `
float specularStrength;

#ifdef USE_SPECULARMAP

	vec4 texelSpecular = texture2D( specularMap, vUv );
	specularStrength = texelSpecular.r;

#else

	specularStrength = 1.0;

#endif
`;