Added join implementation

src/collections/IReadOnlyObservableCollection.ts

@@ -93,6 +93,12 @@ export interface IReadOnlyObservableCollection<TItem> extends Iterable<TItem>, I
     findLast<TResult extends TItem>(callback: (item: TItem, index: number, colleciton: this) => item is TResult): TResult | undefined;
     findLast<TResult extends TItem, TContext>(callback: (this: TContext, item: TItem, index: number, colleciton: this) => item is TResult, thisArg: TContext): TResult | undefined;
 
+    /**
+     * Merges the current collection with the given [Array](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array) and returns a new JavaScript [Array](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array).
+     * @param items The items to concatenate.
+     * @returns Returns a new [Array](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array) containing the items of this collection followed by the items in the provided [Array](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array).
+     * @see [Array.concat](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/concat)
+     */
     concat(...items: readonly (TItem | readonly TItem[])[]): TItem[];
 
     map<TResult>(callback: (item: TItem, index: number, colleciton: this) => TResult): TResult[];
@@ -105,6 +111,12 @@ export interface IReadOnlyObservableCollection<TItem> extends Iterable<TItem>, I
 
     slice(start?: number, end?: number): TItem[];
 
+    /**
+     * Aggregates the contained items into a {@link String} placing the provided `separator` between them.
+     * @param separator The separator used to insert between items when aggregating them into a {@link String}.
+     * @returns The aggregated items as a {@link String}.
+     * @see [Array.join](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/join)
+     */
     join(separator?: string): string;
 
     some(callback: (item: TItem, index: number, collection: this) => boolean): boolean;

src/collections/ObservableCollection.ts

@@ -7,7 +7,8 @@ import { ReadOnlyObservableCollection } from './ReadOnlyObservableCollection';
  * @see [Array](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array)
  */
 export class ObservableCollection<TItem> extends ReadOnlyObservableCollection<TItem> implements IObservableCollection<TItem> {
-    /** Initializes a new instance of the {@link ObservableCollection} class.
+    /**
+     * Initializes a new instance of the {@link ObservableCollection} class.
      * @param items The items to initialize the collection with.
      */
     public constructor(...items: readonly TItem[]) {
@@ -40,15 +41,17 @@ export class ObservableCollection<TItem> extends ReadOnlyObservableCollection<TI
         return super.push(...items);
     }
 
-    /** Removes the last element from the collection and returns it. If the collection is empty, `undefined` is returned.
+    /**
+     * Removes the last element from the collection and returns it. If the collection is empty, `undefined` is returned.
      * @returns The last element in the collection that was removed.
      * @see [Array.pop](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/pop)
      */
     public pop(): TItem | undefined {
         return super.pop();
     }
 
-    /** Inserts new elements at the start of the collection, and returns the new length of the collection.
+    /**
+     * Inserts new elements at the start of the collection, and returns the new length of the collection.
      * @param items Elements to insert at the start of the collection.
      * @returns The new length of the collection.
      * @see [Array.unshift](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/unshift)
@@ -76,7 +79,8 @@ export class ObservableCollection<TItem> extends ReadOnlyObservableCollection<TI
         return super.get(index);
     }
 
-    /** Sets the provided item at the provided index.
+    /**
+     * Sets the provided item at the provided index.
      * @param index The index to which to set the item.
      * @param item The item to set.
      * @throws {@link RangeError} when the index is outside the bounds of the collection.
@@ -85,7 +89,8 @@ export class ObservableCollection<TItem> extends ReadOnlyObservableCollection<TI
         super.set(index, item);
     }
 
-    /** Removes and/or adds elements to the collection and returns the deleted elements.
+    /**
+     * Removes and/or adds elements to the collection and returns the deleted elements.
      * @param start The zero-based location in the collection from which to start removing elements.
      * @param deleteCount The number of elements to remove.
      * @param items The items to insert at the given start location.

src/collections/ReadOnlyObservableCollection.ts

@@ -206,7 +206,7 @@ export class ReadOnlyObservableCollection<TItem> extends ViewModel implements IR
      * @see [Array.join](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/join)
      */
     public join(separator?: string): string {
-        throw new Error('Method not implemented.');
+        return Array.from(this).join(separator);
     }
 
     /**

src/collections/tests/ObservableCollection/ObservableCollection.join.tests.ts

@@ -0,0 +1,64 @@
+import { ObservableCollection } from '../../ObservableCollection';
+import { testBlankMutatingOperation } from './common';
+
+describe('ObserableCollection.join', (): void => {
+    it('joining an empty collection returns an empty string', (): void => {
+        testBlankMutatingOperation<number>({
+            initialState: [],
+
+            applyOperation: collection => collection.join()
+        });
+    });
+
+    it('joining an empty collection with separator returns an empty string', (): void => {
+        testBlankMutatingOperation<number>({
+            initialState: [],
+
+            applyOperation: collection => collection.join('-')
+        });
+    });
+
+    it('joining a collection returns a string containing the string representations of each element in one string separated by comma', (): void => {
+        testBlankMutatingOperation<unknown>({
+            initialState: [null, undefined, 1, 'A', { prop: 'value' }, new Date(), ObservableCollection],
+
+            applyOperation: collection => collection.join()
+        });
+    });
+
+    it('joining a collection with separator returns a string containing the string representations of each element in one string separated by the separator', (): void => {
+        testBlankMutatingOperation<unknown>({
+            initialState: [null, undefined, 1, 'A', { prop: 'value' }, new Date(), ObservableCollection],
+
+            applyOperation: collection => collection.join('-')
+        });
+    });
+
+    it('joining a collection with undefined separator returns a string containing the string representations of each element in one string separated by comma', (): void => {
+        testBlankMutatingOperation<unknown>({
+            initialState: [null, undefined, 1, 'A', { prop: 'value' }, new Date(), ObservableCollection],
+
+            applyOperation: collection => collection.join(undefined)
+        });
+    });
+
+    it('joining a collection with null separator returns a string containing the string representations of each element in one string', (): void => {
+        testBlankMutatingOperation<unknown>({
+            initialState: [null, undefined, 1, 'A', { prop: 'value' }, new Date(), ObservableCollection],
+
+            applyOperation: collection => collection.join(null)
+        });
+    });
+
+    it('calling join while iterating will not break iterators', (): void => {
+        expect(
+            () => {
+                const observableCollection = new ObservableCollection<number>(1, 2, 3);
+
+                for (const _ of observableCollection)
+                    observableCollection.join();
+            })
+            .not
+            .toThrow();
+    });
+});