Convert function with node style callback into a one that returns a Promise

promisify.js

var slice = Array.prototype.slice;

function promisify(f) {
    return function() {
        // Save arguments and promote to real Array.
        var args = slice.call(arguments);

        return new Promise(function(resolve, reject) {
            // Add our callback to argument list.
            args.push(function(err, result) {
                if (err) {
                    reject(err);
                }
                else if (arguments.length == 2) {
                    resolve(result);
                }
                else {
                    // Promises are fulfilled with a single value, so pack all
                    // arguments into an Array (except the undefined error).
                    var results = slice.call(arguments, 1);
                    resolve(results);
                }
            })
        
            f.apply(f, args);
        });
    }
}