Node.js variable use outside of function

Question

I'm trying to make it so that I can pass my trends variable from its function into a renderer for my Pug template, and I can't seem to do it.

var express = require('express');                                                                                                                                   
var router = express.Router();                                                                                                                                      
var googleTrends = require('google-trends-api');                                                                                                                    
var auth = require('http-auth');                                                                                                                                    
var ustrends;                                                                                                                                                       
var uktrends;                                                                                                                                                       
const Console = require('console').Console;                                                                                                                         
var basic = auth.basic({                                                                                                                                            
    realm: "Web."                                                                                                                                               
}, function (username, password, callback) { // Custom authentication method.                                                                                   
    callback(username === "user" && password === "pass");                                                                                                       
   }                                                                                                                                                               
);                                                                                                                                                                  

var find = ',';                                                                                                                                                     
var regex = new RegExp(find, 'g');                                                                                                                                  


googleTrends.hotTrends('US').then(function(trends){                                                                                                                 
ustrends = trends                                                                                                                                               
});                                                                                                                                                                 

googleTrends.hotTrends('EU').then(function(trends1) {                                                                                                               
uktrends = trends1                                                                                                                                              
});                                                                                                                                                                 

console.log(ustrends);                                                                                                                                              
/* GET home page. */                                                                                                                                                


router.get('/', auth.connect(basic), function(req, res, next) {                                                                                                     
res.render('index', {trends: ustrends.toString().replace(regex, ", "), trends1: uktrends.toString().replace(regex, ", "), title: 'Trends in the U.S & U.K'});   

});                                                                                                                                                                 

module.exports = router;                                                                                                                                                                

As you can see, I'm trying to pass the "ustrends" and "uktrends" variables into the renderer. Any help is appreciated.

Answer 1

Remember that hotTrends will return a promise, as it's getting results from Google's API. Since the renderer is outside of the callbacks wherein ustrends and uktrends are set to values, there's no guarantee these values will be set prior to the renderer being called.

You could use several nested callbacks, but that would lead to some code pushed pretty far to the right; I recommend the async library, which has a function called series that allows you to pass in 1) an array of functions to be executed in order and 2) a callback that will be executed after the functions have completed that takes an error if there was one and the result of the functions as an argument. In the snippet below, the trends API returns results prior to the renderer being called:

async.series([
    function(cb) {
        googleTrends.hotTrends('US').then(function(trends){                                                                                                                 
            ustrends = trends;
            cb();
        })
    },
    function(cb) {
        googleTrends.hotTrends('EU').then(function(trends1) {                                                                                                               
            uktrends = trends1;
            cb();                                                                                                                                              
        }); 
    }
], function(err, results) {
    /* handle errors, do rendering stuff */
})