add trackBy function

src/lib/listing/models/listable.ts

@@ -1,5 +1,6 @@
-export interface IListable {
-    readonly id: string;
+import { ITrackable } from './trackable';
+
+export interface IListable extends ITrackable {
     readonly searchKey: string;
     readonly routerLink?: string;
 }

src/lib/listing/models/trackable.ts

@@ -0,0 +1,3 @@
+export interface ITrackable {
+    readonly id: string;
+}

src/lib/utils/functions.ts

@@ -1,4 +1,5 @@
 import { tap } from 'rxjs/operators';
+import { ITrackable } from '../listing/models/trackable';
 
 export function capitalize(value: string): string {
     if (!value) {
@@ -25,3 +26,7 @@ export function toNumber(str: string): number {
         return 0;
     }
 }
+
+export function trackBy() {
+    return <T extends ITrackable>(index: number, item: T): string => item.id;
+}