var is undefined if using var keyword node - mysql

Question

Building a small MVC. When I'm receiving results back from my model, the variable that I'm using to send to my view is undefined if I use the "var" keyword. If I don't use the keyword the object comes through just fine. What is happening?
Controller
const homeModel = require('../models/homeModel.js');
exports.index = function(req, res){
homeModel.getAllStores(function (err, res) {
if (err) return err;
stores = res; // Works
var stores = res // Undefined
})
console.log(stores);
res.render('home', {stores: stores});
}
Here is the Model
const db = require('../db.js');
exports.getAllStores = function(done) {
db.query('select * from stores;', (err, rows) => {
if (err) return done(err);
let resultJson = JSON.stringify(rows);
resultJson = JSON.parse(resultJson);
return done(null, resultJson);
})
}

Answer 1

You need to move the declaration of stores to the function enclosing homeModel.getAllStores(). This is because JavaScript is function (lexically) scoped, so a variable will be scoped to the nearest enclosing function. You can read more about how variables that are declared using var work on MDN.
In Node.js, if you don't provide the var keyword before your variable then it is globally scoped to the module in which it is running, this is why console.log(stores) works when you use stores = res and not var stores = res.
To properly scope your variable using var, just move your declaration to the function being exported.
Additionally, your console.log() and res.render() calls are occurring before the callback function for homeModel.getAllStores() is executed and setting stores = res. Since res.render() and console.log() will only work as expected within the callback to homeModel.getAllStores() you can simplify index() and the callback to homeModel.getAllStores().
const homeModel = require('../models/homeModel.js')
exports.index = (req, res) => {
return homeModel.getAllStores((err, stores) => {
if (err) {
throw err
}
console.log(stores)
return res.render('home', {stores})
})
}
You could also use util.promisify() and async/await to write this a little more straightforward.
const {promisify} = require('util')
const getAllStores = promisify(require('../models/homeModel').getAllStores)
const index = async (req, res) => {
let stores
try {
stores = await getAllStores()
} catch (err) {
console.error(err)
return res.sendStatus(500)
}
return res.render('home', {stores})
}
module.exports = {index}
Here is an example with Promise.all() waiting for the results from multiple queries with a hypothetical UserModel with getAllUsers() that works identically to homeModel.getAllStores() but queries a users table.
const {promisify} = require('util')
const getAllUsers = promisify(require('../models/userModel').getAllUsers)
const getAllStores = promisify(require('../models/homeModel').getAllStores)
const index = async (req, res) => {
let queryResults
try {
queryResults = await Promise.all([getAllStores, getAllUsers])
} catch (err) {
console.error(err)
return res.sendStatus(500)
}
let [stores, users] = queryResults
return res.render('home', {stores, users})
}
module.exports = {index}