Refactor server-side code for clarity

src/server.js

@@ -12,6 +12,7 @@
  * https://developer.spotify.com/web-api/authorization-guide/#authorization_code_flow
  */
 
+'use strict';
 var express = require('express'); // Express web server framework
 var request = require('request'); // "Request" library
 var querystring = require('querystring');
@@ -44,7 +45,7 @@ var stateKey = 'spotify_auth_state';
 var app = express();
 
 if (environment == 'prod' || environment == 'production') {
-    app.use(enforce.HTTPS({ trustProtoHeader: true }));
+    app.use(enforce.HTTPS({trustProtoHeader: true}));
 }
 
 app.use(express.static(__dirname + '/public'))
@@ -189,10 +190,7 @@ app.get('/pl/:uId/:plId', function (req, res) {
             var i = index + 1;
             while (i < array.length) {
                 var other = array[i];
-                if (item.track.id == other.track.id) {
+                if (areDups(item.track, other.track)) {
-                    dups.push(item);
-                }
-                else if (item.track.name.toLowerCase() == other.track.name.toLowerCase() && item.track.artists[0].id == other.track.artists[0].id) {
                     dups.push(item);
                 }
                 i++;
@@ -206,6 +204,49 @@ app.get('/pl/:uId/:plId', function (req, res) {
     });
 });
 
+/**
+ * @typedef {Object} track
+ * @property {number} id
+ * @property {string} name
+ * @property {[artist]} artists
+ */
+
+/**
+ * @typedef {Object} artist
+ * @property {number} id
+ * @property {string} name
+ */
+
+/**
+ * Check if two tracks are duplicates
+ * @param {track} track1
+ * @param {track} track2
+ * @return {boolean} True if the two are duplicates. False otherwise
+ */
+function areDups(track1, track2) {
+    var name1 = track1.name.toLowerCase();
+    var name2 = track2.name.toLowerCase();
+    if (track1.id == track2.id) {
+        return true;
+    }
+    else if (haveCommonArtist(track1, track2)) {
+        if (name1 == name2) {
+            return true;
+        }
+    }
+    return false;
+}
+
+/**
+ * Check if two tracks have at least one artist in common based on artist id.
+ * @param {track} track1
+ * @param {track} track2
+ * @return {boolean} True if they have at least one artist in common. False otherwise.
+ */
+function haveCommonArtist(track1, track2) {
+    return track1.artists[0].id == track2.artists[0].id;
+}
+
 function getAllPages(authOptions, data, callback) {
     request.get(authOptions, function (error, response, body) {
         if (!error && response.statusCode === 200) {