diff --git a/src/scripting_api/aform.js b/src/scripting_api/aform.js
index 17f3f1aea..81639bdd2 100644
--- a/src/scripting_api/aform.js
+++ b/src/scripting_api/aform.js
@@ -368,8 +368,8 @@ class AForm {
 
   AFSimple_Calculate(cFunction, cFields) {
     const actions = {
-      AVG: args => args.reduce((acc, value) => acc + value, 0) / args.length,
-      SUM: args => args.reduce((acc, value) => acc + value, 0),
+      AVG: args => Math.sumPrecise(args) / args.length,
+      SUM: args => Math.sumPrecise(args),
       PRD: args => args.reduce((acc, value) => acc * value, 1),
       MIN: args => Math.min(...args),
       MAX: args => Math.max(...args),
diff --git a/src/scripting_api/app_utils.js b/src/scripting_api/app_utils.js
index b46f8ea26..e4378ebf7 100644
--- a/src/scripting_api/app_utils.js
+++ b/src/scripting_api/app_utils.js
@@ -26,8 +26,39 @@ function serializeError(error) {
   return { command: "error", value };
 }
 
+// Helpers for simple `Map.prototype.getOrInsertComputed()` invocations,
+// to avoid duplicate function creation.
+const makeArr = () => [];
+const makeMap = () => new Map();
+
+if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+  // TODO: Remove this once `Math.sumPrecise` is supported in QuickJS.
+  //
+  // Note that this isn't a "proper" polyfill, but since we're only using it to
+  // replace `Array.prototype.reduce()` invocations it should be fine.
+  if (typeof Math.sumPrecise !== "function") {
+    Math.sumPrecise = function (numbers) {
+      return numbers.reduce((a, b) => a + b, 0);
+    };
+  }
+
+  // TODO: Remove this once `Map.prototype.getOrInsertComputed` is supported in
+  // QuickJS.
+  if (typeof Map.prototype.getOrInsertComputed !== "function") {
+    // eslint-disable-next-line no-extend-native
+    Map.prototype.getOrInsertComputed = function (key, callbackFn) {
+      if (!this.has(key)) {
+        this.set(key, callbackFn(key));
+      }
+      return this.get(key);
+    };
+  }
+}
+
 export {
   FORMS_VERSION,
+  makeArr,
+  makeMap,
   serializeError,
   USERACTIVATION_CALLBACKID,
   USERACTIVATION_MAXTIME_VALIDITY,
diff --git a/src/scripting_api/doc.js b/src/scripting_api/doc.js
index 9e20f6a9e..5b9c66d2b 100644
--- a/src/scripting_api/doc.js
+++ b/src/scripting_api/doc.js
@@ -13,10 +13,10 @@
  * limitations under the License.
  */
 
+import { makeArr, makeMap, serializeError } from "./app_utils.js";
 import { createActionsMap } from "./common.js";
 import { PDFObject } from "./pdf_object.js";
 import { PrintParams } from "./print_params.js";
-import { serializeError } from "./app_utils.js";
 import { ZoomType } from "./constants.js";
 
 const DOC_EXTERNAL = false;
@@ -32,6 +32,10 @@ class InfoProxyHandler {
 }
 
 class Doc extends PDFObject {
+  #pageActions = null;
+
+  #otherPageActions = null;
+
   constructor(data) {
     super(data);
 
@@ -96,11 +100,9 @@ class Doc extends PDFObject {
     this._zoom = data.zoom || 100;
     this._actions = createActionsMap(data.actions);
     this._globalEval = data.globalEval;
-    this._pageActions = null;
     this._userActivation = false;
     this._disablePrinting = false;
     this._disableSaving = false;
-    this._otherPageActions = null;
   }
 
   _initActions() {
@@ -170,14 +172,14 @@ class Doc extends PDFObject {
 
   _dispatchPageEvent(name, actions, pageNumber) {
     if (name === "PageOpen") {
-      this._pageActions ||= new Map();
-      if (!this._pageActions.has(pageNumber)) {
-        this._pageActions.set(pageNumber, createActionsMap(actions));
+      this.#pageActions ??= new Map();
+      if (!this.#pageActions.has(pageNumber)) {
+        this.#pageActions.set(pageNumber, createActionsMap(actions));
       }
       this._pageNum = pageNumber - 1;
     }
 
-    for (const acts of [this._pageActions, this._otherPageActions]) {
+    for (const acts of [this.#pageActions, this.#otherPageActions]) {
       actions = acts?.get(pageNumber)?.get(name);
       if (actions) {
         for (const action of actions) {
@@ -212,27 +214,16 @@ class Doc extends PDFObject {
     const po = field.obj._actions.get("PageOpen");
     const pc = field.obj._actions.get("PageClose");
     if (po || pc) {
-      this._otherPageActions ||= new Map();
-      let actions = this._otherPageActions.get(field.obj._page + 1);
-      if (!actions) {
-        actions = new Map();
-        this._otherPageActions.set(field.obj._page + 1, actions);
-      }
+      this.#otherPageActions ??= new Map();
+      const actions = this.#otherPageActions.getOrInsertComputed(
+        field.obj._page + 1,
+        makeMap
+      );
       if (po) {
-        let poActions = actions.get("PageOpen");
-        if (!poActions) {
-          poActions = [];
-          actions.set("PageOpen", poActions);
-        }
-        poActions.push(...po);
+        actions.getOrInsertComputed("PageOpen", makeArr).push(...po);
       }
       if (pc) {
-        let pcActions = actions.get("PageClose");
-        if (!pcActions) {
-          pcActions = [];
-          actions.set("PageClose", pcActions);
-        }
-        pcActions.push(...pc);
+        actions.getOrInsertComputed("PageClose", makeArr).push(...pc);
       }
     }
   }