google-closure-compiler-js webpack/gulp plugin externs issue quick fix

gistfile.js

// add following code into webpack.config.js or gulpfile.js or webpack/gulp plugin file
// sample options
var options = {
	languageIn: 'ECMASCRIPT6',
	languageOut: 'ECMASCRIPT5',
	compilationLevel: 'ADVANCED',
	warningLevel: 'VERBOSE',
	externs: ['externs.js']
}

function toArray(arg) {
	if (typeof arg === 'string') {
		return [arg];
	} else if (arg) {
		return arg;
	} else {
		return [];
	}
}

function readExternFile(extern) {
	if ('string' === typeof extern || 
		'object' === typeof extern && undefined === extern.src && 'string' === typeof extern.path) {
		var newExtern = {
			src: '',
			path: 'string' === typeof extern? extern: extern.path
		};
		fs.readFile(newExtern.path, 'utf8', (err, src) => {
			if (err) {
				throw new Error(err);  
			}
			else {
				newExtern.src = src;		
			}
		}); 
		return newExtern;	  
	}
	else {
		return extern;
	}
}

function translateOptions(options) {
	var externs = options.externs;
	if (externs) {
		externs = toArray(externs);
		options.externs = externs.map(extern => readExternFile(extern));
	}
}

// we have to translate 'externs' content, 
// due to google closure compiler's webpack plugin doesn't support extern path string(OK with 'src' content) 
translateOptions(options);

var compiler = new ClosureCompiler({options: options});