from promise.js

/**
 * Takes an array of promises and returns a promise that that is fulfilled once all
 * the promises in the array are fulfilled
 * @param group  The array of promises
 * @return the promise that is fulfilled when all the array is fulfilled
 */exports.group = function(group){
	var deferred = defer(); // this function is assigned process.Promise
	if(!(group instanceof Array)){
		group = Array.prototype.slice.call(arguments);
	}
	var fulfilled, length = group.length;
	var results = [];
	group.forEach(function(promise, index){
		exports.when(promise, function(value){
			results[index] = value;
			fulfilled++;
			if(fulfilled === length){
				deferred.resolve(results);
			}
		},
		deferred.reject);
	});
	return deferred.promise;
};