Support PDF.js Map upsert in current browsers

static/reader.js

@@ -3,6 +3,14 @@ const PDFJS_WORKER_URL = "/static/vendor/pdf.worker.min.mjs";
 const EPUB_URL = "/static/vendor/epub.min.js";
 const MARKED_URL = "/static/vendor/marked.min.js";
 const PURIFY_URL = "/static/vendor/purify.min.js";
+if (!Map.prototype.getOrInsertComputed) {
+  Map.prototype.getOrInsertComputed = function(key, callback) {
+    if (this.has(key)) return this.get(key);
+    const value = callback(key);
+    this.set(key, value);
+    return value;
+  };
+}
 const params = new URLSearchParams(location.search);
 const sourceUrl = params.get("url") || "";
 const downloadUrl = params.get("download") || sourceUrl;