Convert app loading message code to TypeScript

ts/setAppLoadingScreenMessage.ts

@@ -0,0 +1,47 @@
+// Copyright 2018-2022 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import type { LocalizerType } from './types/Util';
+
+const DISPLAY_THRESHOLD = 3000; // milliseconds
+const SELECTOR = '.app-loading-screen .message';
+
+let timeout: null | ReturnType<typeof setTimeout>;
+let targetString: string;
+let didTimeout = false;
+
+const clear = () => {
+  if (timeout) {
+    clearTimeout(timeout);
+    timeout = null;
+  }
+};
+
+export function setAppLoadingScreenMessage(
+  loadingText: undefined | string,
+  i18n: LocalizerType
+): () => void {
+  const message = document.querySelector<HTMLElement>(SELECTOR);
+  if (!message) {
+    return clear;
+  }
+
+  targetString = loadingText || i18n('optimizingApplication');
+
+  message.innerText = didTimeout ? targetString : i18n('loading');
+
+  if (timeout) {
+    return clear;
+  }
+
+  timeout = setTimeout(() => {
+    didTimeout = true;
+    const innerMessage = document.querySelector<HTMLElement>(SELECTOR);
+    if (!innerMessage) {
+      return;
+    }
+    innerMessage.innerText = targetString;
+  }, DISPLAY_THRESHOLD);
+
+  return clear;
+}