Sort by Disc Number + improve sort-by pipe algo

dashboard/src/app/object/song.ts

@@ -4,4 +4,5 @@ export class Song {
     'Play Count': number;
     Album: string;
     'Track Number': number; // TODO Default property
+    'Disc Number': number;
 }

dashboard/src/app/song-table/song-table.component.html

@@ -14,7 +14,7 @@
     </tr>
   </thead>
   <tbody>
-    <tr *ngFor="let song of songs | sortBy : 'Year':'Album':'Track Number'">
+    <tr *ngFor="let song of songs | sortBy : 'Year':'Album':'Disc Number':'Track Number'">
       <td>{{song['Track Number'] ? (("0" + song['Track Number']).slice(-2)) : "--"}}</td>
       <td>{{song.Name}}</td>
       <td>

dashboard/src/app/song-table/song-table.component.ts

@@ -49,7 +49,7 @@ export class SongTableComponent {
   }
 
   sort(): void {
-    this.songs = new SortByPipe().transform(this.songs, 'Year', 'Album', 'Track Number', 'Play Count');
+    this.songs = new SortByPipe().transform(this.songs, 'Year', 'Album', 'Disc Number', 'Track Number', 'Play Count');
     this.sortable = false;
   }
 

dashboard/src/app/sort-by.pipe.ts

@@ -5,9 +5,25 @@ import { Pipe, PipeTransform } from '@angular/core';
 })
 export class SortByPipe implements PipeTransform {
 
+  /**
+   * Sort an array by multiple elements.
+   *
+   * Example to sort an array respectively by Year, Album, Disc Number, etc.
+   * Typescript usage:
+   * <pre>
+   * new SortByPipe().transform(this.songs, 'Year', 'Album', 'Disc Number', 'Track Number', 'Play Count');
+   * </pre>
+   * In HTML usage:
+   * <pre>
+   * <tr *ngFor="let song of songs | sortBy : 'Year':'Album':'Disc Number':'Track Number'">
+   * </pre>
+   *
+   * @param array array to sort
+   * @param args elements used to sort in oder
+   */
   transform(array: Array<any>, ...args: any[]): Array<any> {
     array.sort((a: any, b: any) => {
-      for (let i = 0; i < array.length; i++) {
+      for (let i = 0; i < args.length; i++) {
         const arg = args[i];
 
         if (a[arg] === undefined && b[arg] !== undefined) { return -1; }