feat(component): add error as value to LetDirective's context (#3380)

Closes #3343 

BREAKING CHANGES:

The `$error` property from `LetDirective`'s view context is
a thrown error or `undefined` instead of `true`/`false`.

BEFORE:

```ts
<p *ngrxLet="obs$; $error as e">{{ e }}</p>
```

- `e` will be `true` when `obs$` emits error event.
- `e` will be `false` when `obs$` emits next/complete event.

AFTER:

```ts
<p *ngrxLet="obs$; $error as e">{{ e }}</p>
```

- `e` will be thrown error when `obs$` emits error event.
- `e` will be `undefined` when `obs$` emits next/complete event.

modules/component/spec/let/let.directive.spec.ts

@@ -47,7 +47,9 @@ class LetDirectiveTestComponent {
 
 @Component({
   template: `
-    <ng-container *ngrxLet="value$; $error as error">{{ error }}</ng-container>
+    <ng-container *ngrxLet="value$; $error as error">{{
+      error === undefined ? 'undefined' : error
+    }}</ng-container>
   `,
 })
 class LetDirectiveTestErrorComponent {
@@ -376,21 +378,25 @@ describe('LetDirective', () => {
   describe('when error', () => {
     beforeEach(waitForAsync(setupLetDirectiveTestComponentError));
 
-    it('should render the error to false if next or complete', () => {
-      letDirectiveTestComponent.value$ = of(1);
+    it('should render undefined when next event is emitted', () => {
+      letDirectiveTestComponent.value$ = new BehaviorSubject(1);
       fixtureLetDirectiveTestComponent.detectChanges();
-      expect(componentNativeElement.textContent).toBe('false');
+      expect(componentNativeElement.textContent).toBe('undefined');
     });
 
-    it('should render the error to true if one occurs', () => {
-      letDirectiveTestComponent.value$ = throwError(
-        () => new Error('error message')
-      );
+    it('should render undefined when complete event is emitted', () => {
+      letDirectiveTestComponent.value$ = EMPTY;
       fixtureLetDirectiveTestComponent.detectChanges();
-      expect(componentNativeElement.textContent).toBe('true');
+      expect(componentNativeElement.textContent).toBe('undefined');
+    });
+
+    it('should render error when error event is emitted', () => {
+      letDirectiveTestComponent.value$ = throwError(() => 'error message');
+      fixtureLetDirectiveTestComponent.detectChanges();
+      expect(componentNativeElement.textContent).toBe('error message');
     });
 
-    it('should call error handler', () => {
+    it('should call error handler when error event is emitted', () => {
       const errorHandler = TestBed.inject(ErrorHandler);
       const error = new Error('ERROR');
       letDirectiveTestComponent.value$ = throwError(() => error);

modules/component/src/let/let.directive.ts

@@ -26,7 +26,7 @@ export interface LetViewContext<T> {
   /**
    * `*ngrxLet="obs$; let e = $error"` or `*ngrxLet="obs$; $error as e"`
    */
-  $error: boolean;
+  $error: any;
   /**
    * `*ngrxLet="obs$; let c = $complete"` or `*ngrxLet="obs$; $complete as c"`
    */
@@ -66,7 +66,7 @@ export interface LetViewContext<T> {
  *   <app-number [number]="n" *ngIf="!e && !c">
  *   </app-number>
  *
- *   <p *ngIf="e">There is an error.</p>
+ *   <p *ngIf="e">There is an error: {{ e }}</p>
  *   <p *ngIf="c">Observable is completed.</p>
  * </ng-container>
  * ```
@@ -94,7 +94,7 @@ export class LetDirective<U> implements OnInit, OnDestroy {
   private readonly viewContext: LetViewContext<U | null | undefined> = {
     $implicit: undefined,
     ngrxLet: undefined,
-    $error: false,
+    $error: undefined,
     $complete: false,
     $suspense: true,
   };
@@ -106,7 +106,7 @@ export class LetDirective<U> implements OnInit, OnDestroy {
     reset: () => {
       this.viewContext.$implicit = undefined;
       this.viewContext.ngrxLet = undefined;
-      this.viewContext.$error = false;
+      this.viewContext.$error = undefined;
       this.viewContext.$complete = false;
       this.viewContext.$suspense = true;
 
@@ -118,14 +118,14 @@ export class LetDirective<U> implements OnInit, OnDestroy {
       this.viewContext.$suspense = false;
 
       if (event.reset) {
-        this.viewContext.$error = false;
+        this.viewContext.$error = undefined;
         this.viewContext.$complete = false;
       }
 
       this.renderMainView();
     },
     error: (event) => {
-      this.viewContext.$error = true;
+      this.viewContext.$error = event.error;
       this.viewContext.$suspense = false;
 
       if (event.reset) {
@@ -144,7 +144,7 @@ export class LetDirective<U> implements OnInit, OnDestroy {
       if (event.reset) {
         this.viewContext.$implicit = undefined;
         this.viewContext.ngrxLet = undefined;
-        this.viewContext.$error = false;
+        this.viewContext.$error = undefined;
       }
 
       this.renderMainView();

projects/ngrx.io/content/guide/component/let.md

@@ -59,7 +59,7 @@ We can track next, error, and complete events:
   <app-number [number]="n" *ngIf="!e && !c">
   </app-number>
 
-  <p *ngIf="e">There is an error.</p>
+  <p *ngIf="e">There is an error: {{ e }}</p>
   <p *ngIf="c">Observable is completed.</p>
 </ng-container>
 ```