fix(badge): duplicate leftover badge after server-side render (#15417)

Fixes badges being left over if the current page was rendered via Angular Universal. This can cause issues, because the ids on the badges can clash.

src/lib/badge/badge.spec.ts

@@ -11,10 +11,12 @@ describe('MatBadge', () => {
   let badgeDebugElement: DebugElement;
 
   beforeEach(fakeAsync(() => {
-    TestBed.configureTestingModule({
-      imports: [MatBadgeModule],
-      declarations: [BadgeTestApp],
-    }).compileComponents();
+    TestBed
+        .configureTestingModule({
+          imports: [MatBadgeModule],
+          declarations: [BadgeTestApp, PreExistingBadge, NestedBadge],
+        })
+        .compileComponents();
 
     fixture = TestBed.createComponent(BadgeTestApp);
     testComponent = fixture.debugElement.componentInstance;
@@ -184,6 +186,20 @@ describe('MatBadge', () => {
     expect(badgeContent.hasAttribute('aria-label')).toBe(false);
   });
 
+  it('should clear any pre-existing badges', () => {
+    const preExistingFixture = TestBed.createComponent(PreExistingBadge);
+    preExistingFixture.detectChanges();
+
+    expect(preExistingFixture.nativeElement.querySelectorAll('.mat-badge-content').length).toBe(1);
+  });
+
+  it('should not clear badge content from child elements', () => {
+    const preExistingFixture = TestBed.createComponent(NestedBadge);
+    preExistingFixture.detectChanges();
+
+    expect(preExistingFixture.nativeElement.querySelectorAll('.mat-badge-content').length).toBe(2);
+  });
+
 });
 
 /** Test component that contains a MatBadge. */
@@ -214,3 +230,27 @@ class BadgeTestApp {
   badgeDescription: string;
   badgeDisabled = false;
 }
+
+
+@Component({
+  template: `
+    <span matBadge="Hello">
+      home
+      <div class="mat-badge-content">Pre-existing badge</div>
+    </span>
+  `
+})
+class PreExistingBadge {
+}
+
+
+@Component({
+  template: `
+    <span matBadge="Hello">
+      home
+      <span matBadge="Hi">Something</span>
+    </span>
+  `
+})
+class NestedBadge {
+}

src/lib/badge/badge.ts

@@ -177,9 +177,12 @@ export class MatBadge extends _MatBadgeMixinBase implements OnDestroy, CanDisabl
     const rootNode = this._renderer || this._document;
     const badgeElement = rootNode.createElement('span');
     const activeClass = 'mat-badge-active';
+    const contentClass = 'mat-badge-content';
 
+    // Clear any existing badges which may have persisted from a server-side render.
+    this._clearExistingBadges(contentClass);
     badgeElement.setAttribute('id', `mat-badge-content-${this._id}`);
-    badgeElement.classList.add('mat-badge-content');
+    badgeElement.classList.add(contentClass);
     badgeElement.textContent = this.content;
 
     if (this._animationMode === 'NoopAnimations') {
@@ -232,4 +235,18 @@ export class MatBadge extends _MatBadgeMixinBase implements OnDestroy, CanDisabl
     }
   }
 
+  /** Clears any existing badges that might be left over from server-side rendering. */
+  private _clearExistingBadges(cssClass: string) {
+    const element = this._elementRef.nativeElement;
+    let childCount = element.children.length;
+
+    // Use a reverse while, because we'll be removing elements from the list as we're iterating.
+    while (childCount--) {
+      const currentChild = element.children[childCount];
+
+      if (currentChild.classList.contains(cssClass)) {
+        element.removeChild(currentChild);
+      }
+    }
+  }
 }