gistfile1.js

var async = require('async')
var final_array = [];

function a(param) {
  console.log("a");
	
  return param * 10; 

}

function b() {
	function c(callback){
		var arr = [];
		for (var i = 1; i <= 3; i++) {
			arr.push(a(i));
		};

		callback(null, arr);		
	};

	function d(arr, callback){
		async.reduce(arr, final_array, function(memo, item, callback){
			callback(memo.push(item));
		}, function(err, results){
			console.log(results);
		})
	}

	async.waterfall([c, d]);
}

b();