Add "go up" arrow

src/page/up-arrow-button/up-arrow-button.ts

@@ -0,0 +1,42 @@
+import { PageElement } from '../page-element';
+import { generate } from './up-arrow-button.html';
+
+export class UpArrowButton extends PageElement {
+  private static readonly defaultTimeToLive = 2000;
+  private static readonly interval = 50;
+  private timeToLive = 0;
+
+  public constructor(private scrollTarget: PageElement, label: string) {
+    super(generate(label));
+
+    this.htmlRoot.addEventListener('click', this.scrollToTop.bind(this));
+
+    setInterval(() => {
+      this.timeToLive = Math.max(0, this.timeToLive - UpArrowButton.interval);
+      if (this.timeToLive == 0) {
+        this.htmlRoot.style.opacity = '0';
+        this.htmlRoot.style.visibility = 'hidden';
+      }
+    }, UpArrowButton.interval);
+  }
+
+  protected initialize() {
+    this.scrollTarget.htmlRoot.addEventListener('scroll', () => {
+      this.timeToLive = UpArrowButton.defaultTimeToLive;
+      this.htmlRoot.style.opacity = '1';
+      this.htmlRoot.style.visibility = 'visible';
+
+      if (this.scrollTarget.htmlRoot.scrollTop == 0) {
+        this.timeToLive = 0;
+      }
+    });
+  }
+
+  private scrollToTop(e: MouseEvent) {
+    if (this.timeToLive > 0) {
+      this.scrollTarget.htmlRoot.scrollTo({ top: 0, left: 0, behavior: 'smooth' });
+    } else {
+      e.preventDefault();
+    }
+  }
+}