NodeJS + TinyURL - adding items to a list

Sorry if this is a stupid question, but how would I go about adding items into a list? So what I've got is a loop that basically runs through and tries to convert all the urls to tinyurls from a web scraper . It still produces an empty list for images_short. I'm not very familiar with nodejs's syntax. Here's a snippet of code, I've put some data in the images_long list:

const TinyURL = require('tinyurl');

var images_long = ['https://hypebeast.imgix.net/http%3A%2F%2Fhypebeast.com%2Fimage%2F2017%2F06%2Fadidas-skateboarding-lucas-premiere-adv-primeknit-khaki-0.jpg?fit=max&fm=pjpg&h=344&ixlib=php-1.1.0&q=90&w=516&s=728297932403d74d2ac1afa5ecdfa97d', 'https://hypebeast.imgix.net/http%3A%2F%2Fhypebeast.com%2Fimage%2F2017%2F06%2Fadidas-nmd-r1-stlt-triple-black-first-look-0.jpg?fit=max&fm=pjpg&h=344&ixlib=php-1.1.0&q=90&w=516&s=918752eba81826e4398950efc69a5141'];
var images_short = [];

for (i = 0; i < 2; i++) {
    TinyURL.shorten(images_long[i], function(res) {
        images_short.push(res[i]);
    });
}

I still get an empty list when I changed images_short.push(res[i]); to images_short.push(res);

res is a string, so just images_short.push(res); will do the trick. Also, you should iterate with respect to the length of the variable you're indexing, and you should var your indexing variable (i):

const TinyURL = require('tinyurl');

var images_long = [
    'https://hypebeast.imgix.net/http%3A%2F%2Fhypebeast.com%2Fimage%2F2017%2F06%2Fadidas-skateboarding-lucas-premiere-adv-primeknit-khaki-0.jpg?fit=max&fm=pjpg&h=344&ixlib=php-1.1.0&q=90&w=516&s=728297932403d74d2ac1afa5ecdfa97d',
    'https://hypebeast.imgix.net/http%3A%2F%2Fhypebeast.com%2Fimage%2F2017%2F06%2Fadidas-nmd-r1-stlt-triple-black-first-look-0.jpg?fit=max&fm=pjpg&h=344&ixlib=php-1.1.0&q=90&w=516&s=918752eba81826e4398950efc69a5141'];
var images_short = [];

for (var i = 0; i < images_long.length; i++) {
    TinyURL.shorten(images_long[i], function(res) {
        images_short.push(res);
    });
}