finish TreeModel implementation and updateMany builder method

src/util/collection/Collection.ts

@@ -12,6 +12,7 @@ type DeterminesEquality<T> = (item: CollectionItem<T>, other: any) => boolean
 type CollectionIndex = number
 type MaybeCollectionIndex = CollectionIndex | undefined
 type ComparisonFunction<T> = (item: CollectionItem<T>, otherItem: CollectionItem<T>) => number
+type Collectable<T> = CollectionItem<T>[] | Collection<T>
 
 import { WhereOperator, applyWhere, whereMatch } from './where'
 import {Awaitable, Awaited, Either, isLeft, Maybe, MethodsOf, MethodType, right, unright} from '../support/types'
@@ -39,6 +40,7 @@ export {
     CollectionIndex,
     MaybeCollectionIndex,
     ComparisonFunction,
+    Collectable,
 }
 
 /**
@@ -61,13 +63,16 @@ class Collection<T> {
      * Filters out undefined items.
      * @param itemOrItems
      */
-    public static normalize<T2>(itemOrItems: (CollectionItem<T2> | undefined)[] | CollectionItem<T2> | undefined): Collection<T2> {
+    public static normalize<T2>(itemOrItems: Collection<T2> | (CollectionItem<T2>)[] | CollectionItem<T2>): Collection<T2> {
+        if ( itemOrItems instanceof Collection ) {
+            return itemOrItems
+        }
+
         if ( !Array.isArray(itemOrItems) ) {
             itemOrItems = [itemOrItems]
         }
 
-        const items = itemOrItems.filter(x => typeof x !== 'undefined') as CollectionItem<T2>[]
-        return new Collection<T2>(items)
+        return new Collection<T2>(itemOrItems)
     }
 
     /**
@@ -1041,6 +1046,8 @@ class Collection<T> {
      * @param reducer
      * @param initialValue
      */
+    reduce<T2>(reducer: KeyReducerFunction<T, T2>, initialValue: T2): T2
+
     reduce<T2>(reducer: KeyReducerFunction<T, T2>, initialValue?: T2): T2 | undefined {
         let currentValue = initialValue
         this.storedItems.forEach((item, index) => {