How to use optional arguments in functions (with optional callback)

 function example( err, optionalA, optionalB, callback ) {
 // retrieve arguments as array
 var args = new Array(arguments.length);
 for(var i = 0; i < args.length; ++i) {
 args[i] = arguments[i];
 };
 
 // first argument is the error object
 // shift() removes the first item from the
 // array and returns it
 err = args.shift();
 // if last argument is a function then its the callback function.
 // pop() removes the last item in the array
 // and returns it
 if (typeof args[args.length-1] === 'function') { 
 callback = args.pop();
 }
 
 // if args still holds items, these are
 // your optional items which you could
 // retrieve one by one like this:
 if (args.length > 0) optionalA = args.shift(); else optionalA = null;
 if (args.length > 0) optionalB = args.shift(); else optionalB = null;
 // continue as usual: check for errors
 if (err) { 
 return callback && callback(err);
 }
 
 // for tutorial purposes, log the optional parameters
 console.log('optionalA:', optionalA);
 console.log('optionalB:', optionalB);
 console.log('callback:', callback);
 /* do your thing */
 }
 // ES6 with shorter, more terse code
 function example(...args) {
 // first argument is the error object
 const err = args.shift();
 // if last argument is a function then its the callback function
 const callback = (typeof args[args.length-1] === 'function') ? args.pop() : null;
 // if args still holds items, these are your optional items which you could retrieve one by one like this:
 const optionalA = (args.length > 0) ? args.shift() : null;
 const optionalB = (args.length > 0) ? args.shift() : null;
 // ... repeat for more items
 if (err && callback) return callback(err);
 /* do your thing */
 }
 // invoke example function with and without optional arguments
 
 example(null, 'AA');
 example(null, function (err) { /* do something */ });
 example(null, 'AA', function (err) {});
 example(null, 'AAAA', 'BBBB', function (err) {});