map and merge arrays

map.js

function map(collection, callback) {
                     var index = -1,
                        length = collection ? collection.length : 0,
                        result = [];
                     
                     if (angular.isArray(collection)) {
                        while (++index < length) {
                            var part = callback(collection[index], index, collection);
                            pushAll(result, part);
                        }
                     } else {
                        collection.forEach(function (value, key, collection) {
                            var part = callback(value, key, collection);
                            pushAll(result, part);
                        });
                     }
                    return result;

                    function pushAll(destination, source) {
                        if (angular.isArray(source)) {
                            source.forEach(function (elem) {
                                destination.push(elem);
                            });
                        } else {
                            destination.push(source);
                        }
                    }
                }