Figuring out scope

photo.js

// Constants
var API_HOST = 'api.instagram.com';
var API_BASE_URL = '/v1';
var CLIENT_ID = 'the id';
var CLIENT_SECRET = 'the secret';

var OAuth= require('oauth').OAuth;
var https = require('https');
var util = require('util');

var photos = [];

// Display the latest popular photos.
var popular = exports.popular = function(req, res) {
  fetchPopular(function(photos) {
    res.render('index', {
      title: 'Instagram',
      photos: photos
    });
  });
}

// Fetch popular photos.
var fetchPopular = function(finish) {
  if (!photos.length) {
    var options = {
      host: API_HOST,
      path: API_BASE_URL + '/media/popular?client_id=' + CLIENT_ID,
    };
    https.get(options, function(res) {
      var data = '';
      res.on('data', function(chunk) {
        data += chunk;
      });
      res.on('end', function() {
        var parsed = JSON.parse(data);
        for (var i in parsed.data) {
          photos.push(new photo(parsed.data[i]));
        }
        finish(photos);
      });
    }).on('error', function(e) {
      console.error(e);
    });
  }
  else {
    finish(photos);
  }
}

// Parse the properties we want from photo data.
var photo = function(data) {
  if (data.caption) {
    this.caption = data.caption.text;
  }
  if (typeof data.images.standard_resolution != 'undefined') {
    this.image = data.images.standard_resolution;
  }
  if (typeof data.images.thumbnail != 'undefined') {
    this.thumb = data.images.thumbnail;
  }
  if (typeof data.link != 'undefined') {
    this.link = data.link;
  }
  if (typeof data.user != 'undefined') {
    this.user = data.user;
  }
};
photo.prototype = {
  render: function() {
    return '<img src="' + this.image.url  + '" width="' + this.image.width  + '" height="' + this.image.height  + '">';
  }
};