diff --git a/lib/utils.js b/lib/utils.js
index ade8309..ed08161 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -252,10 +252,11 @@ const trim = (str) => str.trim ?
  * If 'obj' is an Object callback will be called passing
  * the value, key, and complete object for each property.
  *
- * @param {Object|Array} obj The object to iterate
+ * @param {Object|Array<unknown>} obj The object to iterate
  * @param {Function} fn The callback to invoke for each item
  *
- * @param {Boolean} [allOwnKeys = false]
+ * @param {Object} [options]
+ * @param {Boolean} [options.allOwnKeys = false]
  * @returns {any}
  */
 function forEach(obj, fn, {allOwnKeys = false} = {}) {
@@ -369,15 +370,26 @@ function merge(/* obj1, obj2, obj3, ... */) {
  * @param {Object} b The object to copy properties from
  * @param {Object} thisArg The object to bind function to
  *
- * @param {Boolean} [allOwnKeys]
+ * @param {Object} [options]
+ * @param {Boolean} [options.allOwnKeys]
  * @returns {Object} The resulting value of object a
  */
 const extend = (a, b, thisArg, {allOwnKeys}= {}) => {
   forEach(b, (val, key) => {
     if (thisArg && isFunction(val)) {
-      a[key] = bind(val, thisArg);
+      Object.defineProperty(a, key, {
+        value: bind(val, thisArg),
+        writable: true,
+        enumerable: true,
+        configurable: true
+      });
     } else {
-      a[key] = val;
+      Object.defineProperty(a, key, {
+        value: val,
+        writable: true,
+        enumerable: true,
+        configurable: true
+      });
     }
   }, {allOwnKeys});
   return a;
@@ -408,7 +420,12 @@ const stripBOM = (content) => {
  */
 const inherits = (constructor, superConstructor, props, descriptors) => {
   constructor.prototype = Object.create(superConstructor.prototype, descriptors);
-  constructor.prototype.constructor = constructor;
+  Object.defineProperty(constructor.prototype, 'constructor', {
+    value: constructor,
+    writable: true,
+    enumerable: false,
+    configurable: true
+  });
   Object.defineProperty(constructor, 'super', {
     value: superConstructor.prototype
   });