Nodejs - Why my variable in class is undefined? - node.js

Question

In crawler.js :
var async = require('async');
var request = require('request');
var cheerio = require('cheerio');
var Helper = require('./helper.js');
var helper = new Helper();
var Crawler = function(){
var me = this;
this.listCategory;
this.getCategory = function(){
//1. not use async : test key input is idMenu
helper.categoryInMenu(1, function(err, result){
me.listCategory = result;
});
//2. use async : test key input is idMenu
async.map([1], helper.categoryInMenu, function(err, result){
me.listCategory = result;
//console.log(result);
});
console.log(me.listCategory); // return undefined
}
}
module.exports = Crawler;
When i set me.listCategory = result; (with result != null) then call listCategory in file main.js or console.log it in file crawler.js.
Display in command is undefined.
File main.js
var Crawler = require('./crawler.js');
var snipper = new Crawler();
snipper.getCategory();
console.log(snipper.listCategory);
I try use async module with function map, it not work.
Maybe, i not set me.listCategory = return of function helper.categoryInMenu.
File helper.js
var db = require('mysql');
var config = require('./configLoader.js');
config.load(__dirname+'/config.json');
var Helper = function(){
var me = this;
this.conn = db.createConnection(config.get('db'));
this.menu = function(callback){
me.conn.query("SELECT * FROM `menu`", function(err, rows){
callback(err, rows);
});
}
// return list category with "input" is a idMenu
this.categoryInMenu = function(idMenu, callback){
me.conn.query("SELECT * FROM `category` WHERE idMenu = ?", idMenu, function(err, rows){
callback(err, rows);
});
}
// return config of Category with "input" is a idCategory (Category)
this.dom = function(idCategory, callback){
me.conn.query("SELECT * FROM `category` WHERE id = ?", idCategory, function(err, rows){
callback(err, JSON.parse(rows[0].dom));
});
}
}
module.exports = Helper;

Answer 1

Map in all languages is used to transform the values of iterable into something else, that is why you should return in the callback function and the result will be new array containing only the returned values. If you need to iterate only, you should use arr.forEach (>ES5) or simple for loop.
Your problem is that async.map is asynchronous which means console.log(me.listCategory); is executed before the actual map.
Array.map is actually ES5 standard so you don't need async module to map arrays, also the native map is synchronous.