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) {});