diff --git a/packages/overlayscrollbars/src/environment/environment.ts b/packages/overlayscrollbars/src/environment/environment.ts
index e956f7c..7161567 100644
--- a/packages/overlayscrollbars/src/environment/environment.ts
+++ b/packages/overlayscrollbars/src/environment/environment.ts
@@ -4,7 +4,7 @@ import {
   style,
   appendChildren,
   clientSize,
-  offset,
+  absoluteCoordinates,
   offsetSize,
   scrollLeft,
   jsAPI,
@@ -45,10 +45,10 @@ const rtlScrollBehavior = (parentElm: HTMLElement, childElm: HTMLElement): { i:
   style(parentElm, { overflowX: strHidden, overflowY: strHidden });
   scrollLeft(parentElm, 0);
 
-  const parentOffset = offset(parentElm);
-  const childOffset = offset(childElm);
+  const parentOffset = absoluteCoordinates(parentElm);
+  const childOffset = absoluteCoordinates(childElm);
   scrollLeft(parentElm, -999); // https://github.com/KingSora/OverlayScrollbars/issues/187
-  const childOffsetAfterScroll = offset(childElm);
+  const childOffsetAfterScroll = absoluteCoordinates(childElm);
   return {
     /**
      * origin direction = determines if the zero scroll position is on the left or right side
diff --git a/packages/overlayscrollbars/src/environment/index.ts b/packages/overlayscrollbars/src/environment/index.ts
index c0f4de3..247d7e7 100644
--- a/packages/overlayscrollbars/src/environment/index.ts
+++ b/packages/overlayscrollbars/src/environment/index.ts
@@ -1 +1,4 @@
+import { Environment } from 'environment/environment';
+
 export * from 'environment/environment';
+export type OSEnvironment = Omit<Environment, 'addListener' | 'removeListener'>;
diff --git a/packages/overlayscrollbars/src/support/compatibility/vendors.ts b/packages/overlayscrollbars/src/support/compatibility/vendors.ts
index c7f3d6c..6ca3b02 100644
--- a/packages/overlayscrollbars/src/support/compatibility/vendors.ts
+++ b/packages/overlayscrollbars/src/support/compatibility/vendors.ts
@@ -81,7 +81,7 @@ export const cssPropertyValue = (property: string, values: string, suffix?: stri
  * Get the requested JS function, object or constructor with vendor prefix if it isn't supported without or undefined if unsupported.
  * @param name The name of the JS function, object or constructor.
  */
-export const jsAPI = (name: string): any => {
+export const jsAPI = <T = any>(name: string): T | undefined => {
   let result: any = jsCache[name] || window[name];
 
   if (hasOwnProperty(jsCache, name)) {
diff --git a/packages/overlayscrollbars/src/support/dom/create.ts b/packages/overlayscrollbars/src/support/dom/create.ts
index d39c5fc..3a704ca 100644
--- a/packages/overlayscrollbars/src/support/dom/create.ts
+++ b/packages/overlayscrollbars/src/support/dom/create.ts
@@ -2,8 +2,15 @@ import { each } from 'support/utils/array';
 import { contents } from 'support/dom/traversal';
 import { removeElements } from 'support/dom/manipulation';
 
+/**
+ * Creates a div DOM node.
+ */
 export const createDiv = (): HTMLDivElement => document.createElement('div');
 
+/**
+ * Creates DOM nodes modeled after the passed html string and returns the root dom nodes as a array.
+ * @param html The html string after which the DOM nodes shall be created.
+ */
 export const createDOM = (html: string): ReadonlyArray<Node> => {
   const createdDiv = createDiv();
   createdDiv.innerHTML = html.trim();
diff --git a/packages/overlayscrollbars/src/support/dom/dimensions.ts b/packages/overlayscrollbars/src/support/dom/dimensions.ts
index da8f56d..a26fb1a 100644
--- a/packages/overlayscrollbars/src/support/dom/dimensions.ts
+++ b/packages/overlayscrollbars/src/support/dom/dimensions.ts
@@ -5,11 +5,18 @@ const zeroObj: WH = {
   h: 0,
 };
 
+/**
+ * Returns the window inner- width and height.
+ */
 export const windowSize = (): WH => ({
   w: window.innerWidth,
   h: window.innerHeight,
 });
 
+/**
+ * Returns the offset- width and height of the passed element. If the element is null the width and height values are 0.
+ * @param elm The element of which the offset- width and height shall be returned.
+ */
 export const offsetSize = (elm: HTMLElement | null): WH =>
   elm
     ? {
@@ -18,6 +25,10 @@ export const offsetSize = (elm: HTMLElement | null): WH =>
       }
     : zeroObj;
 
+/**
+ * Returns the client- width and height of the passed element. If the element is null the width and height values are 0.
+ * @param elm The element of which the client- width and height shall be returned.
+ */
 export const clientSize = (elm: HTMLElement | null): WH =>
   elm
     ? {
@@ -26,4 +37,8 @@ export const clientSize = (elm: HTMLElement | null): WH =>
       }
     : zeroObj;
 
+/**
+ * Returns the BoundingClientRect of the passed element.
+ * @param elm The element of which the BoundingClientRect shall be returned.
+ */
 export const getBoundingClientRect = (elm: HTMLElement): DOMRect => elm.getBoundingClientRect();
diff --git a/packages/overlayscrollbars/src/support/dom/manipulation.ts b/packages/overlayscrollbars/src/support/dom/manipulation.ts
index adc96c2..fc8831f 100644
--- a/packages/overlayscrollbars/src/support/dom/manipulation.ts
+++ b/packages/overlayscrollbars/src/support/dom/manipulation.ts
@@ -89,9 +89,9 @@ export const removeElements = (nodes: NodeCollection): void => {
   if (isArrayLike(nodes)) {
     each(from(nodes), (e) => removeElements(e));
   } else if (nodes) {
-    const { parentNode } = nodes;
-    if (parentNode) {
-      parentNode.removeChild(nodes);
+    const parentElm = parent(nodes);
+    if (parentElm) {
+      parentElm.removeChild(nodes);
     }
   }
 };
diff --git a/packages/overlayscrollbars/src/support/dom/offset.ts b/packages/overlayscrollbars/src/support/dom/offset.ts
index 16e54d9..854c49d 100644
--- a/packages/overlayscrollbars/src/support/dom/offset.ts
+++ b/packages/overlayscrollbars/src/support/dom/offset.ts
@@ -6,7 +6,11 @@ const zeroObj: XY = {
   y: 0,
 };
 
-export const offset = (elm: HTMLElement | null): XY => {
+/**
+ * Returns the offset- left and top coordinates of the passed element relative to the document. If the element is null the top and left values are 0.
+ * @param elm The element of which the offset- top and left coordinates shall be returned.
+ */
+export const absoluteCoordinates = (elm: HTMLElement | null): XY => {
   const rect = elm ? getBoundingClientRect(elm) : 0;
   return rect
     ? {
@@ -16,7 +20,11 @@ export const offset = (elm: HTMLElement | null): XY => {
     : zeroObj;
 };
 
-export const position = (elm: HTMLElement | null): XY =>
+/**
+ * Returns the offset- left and top coordinates of the passed element. If the element is null the top and left values are 0.
+ * @param elm The element of which the offset- top and left coordinates shall be returned.
+ */
+export const offsetCoordinates = (elm: HTMLElement | null): XY =>
   elm
     ? {
         x: elm.offsetLeft,
diff --git a/packages/overlayscrollbars/src/support/dom/style.ts b/packages/overlayscrollbars/src/support/dom/style.ts
index cc00b06..9913163 100644
--- a/packages/overlayscrollbars/src/support/dom/style.ts
+++ b/packages/overlayscrollbars/src/support/dom/style.ts
@@ -31,6 +31,11 @@ const setCSSVal = (elm: HTMLElement | null, prop: string, val: string | number):
   } catch (e) {}
 };
 
+/**
+ * Gets or sets the passed styles to the passed element.
+ * @param elm The element to which the styles shall be applied to / be read from.
+ * @param styles The styles which shall be set or read.
+ */
 export function style(elm: HTMLElement | null, styles: CssStyles): void;
 export function style(elm: HTMLElement | null, styles: string): string;
 export function style(elm: HTMLElement | null, styles: Array<string> | string): { [key: string]: string };
@@ -54,10 +59,18 @@ export function style(elm: HTMLElement | null, styles: CssStyles | Array<string>
   each(keys(styles), (key) => setCSSVal(elm, key, styles[key]));
 }
 
+/**
+ * Hides the passed element (display: none).
+ * @param elm The element which shall be hidden.
+ */
 export const hide = (elm: HTMLElement | null): void => {
   style(elm, { display: 'none' });
 };
 
+/**
+ * Shows the passed element (display: block).
+ * @param elm The element which shall be shown.
+ */
 export const show = (elm: HTMLElement | null): void => {
   style(elm, { display: 'block' });
 };
diff --git a/packages/overlayscrollbars/src/support/dom/traversal.ts b/packages/overlayscrollbars/src/support/dom/traversal.ts
index 41a601d..df61979 100644
--- a/packages/overlayscrollbars/src/support/dom/traversal.ts
+++ b/packages/overlayscrollbars/src/support/dom/traversal.ts
@@ -2,6 +2,11 @@ import { each, from } from 'support/utils/array';
 
 const elementIsVisible = (elm: HTMLElement): boolean => !!(elm.offsetWidth || elm.offsetHeight || elm.getClientRects().length);
 
+/**
+ * Find all elements with the passed selector, outgoing (and including) the passed element or the document if no element was provided.
+ * @param selector The selector which has to be searched by.
+ * @param elm The element from which the search shall be outgoing.
+ */
 export const find = (selector: string, elm?: Element | null): ReadonlyArray<Element> => {
   const arr: Array<Element> = [];
 
@@ -12,8 +17,18 @@ export const find = (selector: string, elm?: Element | null): ReadonlyArray<Elem
   return arr;
 };
 
+/**
+ * Find the first element with the passed selector, outgoing (and including) the passed element or the document if no element was provided.
+ * @param selector The selector which has to be searched by.
+ * @param elm The element from which the search shall be outgoing.
+ */
 export const findFirst = (selector: string, elm?: Element | null): Element | null => (elm || document).querySelector(selector);
 
+/**
+ * Determines whether the passed element is matching with the passed selector.
+ * @param elm The element which has to be compared with the passed selector.
+ * @param selector The selector which has to be compared with the passed element. Additional selectors: ':visible' and ':hidden'.
+ */
 export const is = (elm: Element | null, selector: string): boolean => {
   if (elm) {
     if (selector === ':visible') {
@@ -29,6 +44,11 @@ export const is = (elm: Element | null, selector: string): boolean => {
   return false;
 };
 
+/**
+ * Returns the children (no text-nodes or comments) of the passed element which are matching the passed selector. An empty array is returned if the passed element is null.
+ * @param elm The element of which the children shall be returned.
+ * @param selector The selector which must match with the children elements.
+ */
 export const children = (elm: Element | null, selector?: string): ReadonlyArray<Element> => {
   const childs: Array<Element> = [];
 
@@ -45,6 +65,14 @@ export const children = (elm: Element | null, selector?: string): ReadonlyArray<
   return childs;
 };
 
+/**
+ * Returns the childNodes (incl. text-nodes or comments etc.) of the passed element. An empty array is returned if the passed element is null.
+ * @param elm The element of which the childNodes shall be returned.
+ */
 export const contents = (elm: Element | null): ReadonlyArray<ChildNode> => (elm ? from(elm.childNodes) : []);
 
+/**
+ * Returns the parent element of the passed element, or null if the passed element is null.
+ * @param elm The element of which the parent element shall be returned.
+ */
 export const parent = (elm: Node | null): Node | null => (elm ? elm.parentElement : null);
diff --git a/packages/overlayscrollbars/src/support/utils/array.ts b/packages/overlayscrollbars/src/support/utils/array.ts
index 3b2248e..e07fe0e 100644
--- a/packages/overlayscrollbars/src/support/utils/array.ts
+++ b/packages/overlayscrollbars/src/support/utils/array.ts
@@ -51,6 +51,10 @@ export function each<T>(
  */
 export const indexOf = <T = any>(arr: Array<T>, item: T, fromIndex?: number): number => arr.indexOf(item, fromIndex);
 
+/**
+ * Creates a shallow-copied Array instance from an array-like or iterable object.
+ * @param arr The object from which the array instance shall be created.
+ */
 export const from = <T = any>(arr: ArrayLike<T>) => {
   if (Array.from) {
     return Array.from(arr);
diff --git a/packages/overlayscrollbars/src/support/utils/object.ts b/packages/overlayscrollbars/src/support/utils/object.ts
index 292f59d..46416bf 100644
--- a/packages/overlayscrollbars/src/support/utils/object.ts
+++ b/packages/overlayscrollbars/src/support/utils/object.ts
@@ -15,12 +15,12 @@ export const hasOwnProperty = (obj: any, prop: string | number | symbol): boolea
 export const keys = (obj: any): Array<string> => (obj ? Object.keys(obj) : []);
 
 // https://github.com/jquery/jquery/blob/master/src/core.js#L116
-export function extend<T, U>(target: T, object1: U): T & U;
-export function extend<T, U, V>(target: T, object1: U, object2: V): T & U & V;
-export function extend<T, U, V, W>(target: T, object1: U, object2: V, object3: W): T & U & V & W;
-export function extend<T, U, V, W, X>(target: T, object1: U, object2: V, object3: W, object4: X): T & U & V & W & X;
-export function extend<T, U, V, W, X, Y>(target: T, object1: U, object2: V, object3: W, object4: X, object5: Y): T & U & V & W & X & Y;
-export function extend<T, U, V, W, X, Y, Z>(
+export function assignDeep<T, U>(target: T, object1: U): T & U;
+export function assignDeep<T, U, V>(target: T, object1: U, object2: V): T & U & V;
+export function assignDeep<T, U, V, W>(target: T, object1: U, object2: V, object3: W): T & U & V & W;
+export function assignDeep<T, U, V, W, X>(target: T, object1: U, object2: V, object3: W, object4: X): T & U & V & W & X;
+export function assignDeep<T, U, V, W, X, Y>(target: T, object1: U, object2: V, object3: W, object4: X, object5: Y): T & U & V & W & X & Y;
+export function assignDeep<T, U, V, W, X, Y, Z>(
   target: T,
   object1?: U,
   object2?: V,
@@ -62,10 +62,8 @@ export function extend<T, U, V, W, X, Y, Z>(
         }
 
         // Never move original objects, clone them
-        target[key] = extend(clone, copy) as any;
-
-        // Don't bring in undefined values
-      } else if (copy !== undefined) {
+        target[key] = assignDeep(clone, copy) as any;
+      } else {
         target[key] = copy;
       }
     });
diff --git a/packages/overlayscrollbars/tests/support/dom/offset.test.ts b/packages/overlayscrollbars/tests/support/dom/offset.test.ts
index 78245a4..42c7f90 100644
--- a/packages/overlayscrollbars/tests/support/dom/offset.test.ts
+++ b/packages/overlayscrollbars/tests/support/dom/offset.test.ts
@@ -1,33 +1,33 @@
 import { isNumber, isPlainObject } from 'support/utils/types';
-import { offset, position } from 'support/dom/offset';
+import { absoluteCoordinates, offsetCoordinates } from 'support/dom/offset';
 
 describe('dom offset', () => {
-  describe('offset', () => {
+  describe('absoluteCoordinates', () => {
     test('DOM element', () => {
-      const result = offset(document.body);
+      const result = absoluteCoordinates(document.body);
       expect(isPlainObject(result)).toBe(true);
       expect(isNumber(result.x)).toBe(true);
       expect(isNumber(result.y)).toBe(true);
     });
 
     test('null', () => {
-      const result = offset(null);
+      const result = absoluteCoordinates(null);
       expect(isPlainObject(result)).toBe(true);
       expect(result.x).toBe(0);
       expect(result.y).toBe(0);
     });
   });
 
-  describe('position', () => {
+  describe('offsetCoordinates', () => {
     test('DOM element', () => {
-      const result = position(document.body);
+      const result = offsetCoordinates(document.body);
       expect(isPlainObject(result)).toBe(true);
       expect(isNumber(result.x)).toBe(true);
       expect(isNumber(result.y)).toBe(true);
     });
 
     test('null', () => {
-      const result = position(null);
+      const result = offsetCoordinates(null);
       expect(isPlainObject(result)).toBe(true);
       expect(result.x).toBe(0);
       expect(result.y).toBe(0);
diff --git a/packages/overlayscrollbars/tests/support/options/validation.test.ts b/packages/overlayscrollbars/tests/support/options/validation.test.ts
index 74e426d..b2087e6 100644
--- a/packages/overlayscrollbars/tests/support/options/validation.test.ts
+++ b/packages/overlayscrollbars/tests/support/options/validation.test.ts
@@ -1,5 +1,5 @@
 import { validate, optionsTemplateTypes as oTypes, OptionsTemplate } from 'support/options';
-import { extend, isEmptyObject } from 'support/utils';
+import { assignDeep, isEmptyObject } from 'support/utils';
 
 type TestOptionsObj = { propA: 'propA'; null: null };
 type TestOptionsEnum = 'A' | 'B' | 'C';
@@ -55,7 +55,7 @@ describe('options validation', () => {
         foreignProp: 'foreign',
         foreignDeep: { a: 'A', b: 'B' },
       };
-      const modifiedOptions = extend({}, options, { nested: foreignObj }, foreignObj);
+      const modifiedOptions = assignDeep({}, options, { nested: foreignObj }, foreignObj);
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -63,14 +63,14 @@ describe('options validation', () => {
     });
 
     test('passed objects arent mutated', () => {
-      const clonedOptions = extend({}, options);
+      const clonedOptions = assignDeep({}, options);
       validate(clonedOptions, template, clonedOptions);
 
       expect(clonedOptions).toEqual(options);
     });
 
     test('passed object isnt returned object', () => {
-      const clonedOptions = extend({}, options);
+      const clonedOptions = assignDeep({}, options);
       const result = validate(clonedOptions, template);
 
       expect(result.validated).not.toBe(clonedOptions);
@@ -86,7 +86,7 @@ describe('options validation', () => {
 
     test('return signle non-object foreign property', () => {
       const foreignObj = { foreignProp: 'foreign' };
-      const modifiedOptions = extend({}, options, foreignObj);
+      const modifiedOptions = assignDeep({}, options, foreignObj);
       const result = validate(modifiedOptions, template);
       const { foreign } = result;
 
@@ -98,7 +98,7 @@ describe('options validation', () => {
         foreignProp: 'foreign',
         foreignDeep: { a: 'A', b: 'B' },
       };
-      const modifiedOptions = extend({}, options, foreignObj);
+      const modifiedOptions = assignDeep({}, options, foreignObj);
       const result = validate(modifiedOptions, template);
       const { foreign } = result;
 
@@ -110,7 +110,7 @@ describe('options validation', () => {
         foreignProp: 'foreign',
         foreignDeep: { a: 'A', b: 'B' },
       };
-      const modifiedOptions = extend({}, options, { nested: foreignObj }, foreignObj);
+      const modifiedOptions = assignDeep({}, options, { nested: foreignObj }, foreignObj);
       const result = validate(modifiedOptions, template);
       const { foreign } = result;
 
@@ -122,7 +122,7 @@ describe('options validation', () => {
 
   describe('diff property return', () => {
     test('one value changed', () => {
-      const modifiedOptions = extend({}, options, { str: 'newvaluetest' });
+      const modifiedOptions = assignDeep({}, options, { str: 'newvaluetest' });
       const result = validate(modifiedOptions, template, options);
       const { validated } = result;
 
@@ -132,7 +132,7 @@ describe('options validation', () => {
     });
 
     test('multiple values changed', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         str: 'newvaluetest',
         nullbool: null,
       });
@@ -147,7 +147,7 @@ describe('options validation', () => {
     });
 
     test('one nested value changed', () => {
-      const modifiedOptions = extend({}, options, { nested: { num: -1293 } });
+      const modifiedOptions = assignDeep({}, options, { nested: { num: -1293 } });
       const result = validate(modifiedOptions, template, options);
       const { validated } = result;
 
@@ -159,7 +159,7 @@ describe('options validation', () => {
     });
 
     test('multiple nested values changed', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: { num: -1293, abc: 'C' },
       });
       const result = validate(modifiedOptions, template, options);
@@ -176,7 +176,7 @@ describe('options validation', () => {
 
     test('various values changed', () => {
       const newFunc = () => {};
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         str: 'newstrvalue',
         func: newFunc,
         abc: 'C',
@@ -206,7 +206,7 @@ describe('options validation', () => {
         foreignDeep: { a: 'A', b: 'B' },
       };
       const newFunc = () => {};
-      const modifiedOptions = extend(
+      const modifiedOptions = assignDeep(
         {},
         options,
         {
@@ -239,7 +239,7 @@ describe('options validation', () => {
 
   describe('value validity', () => {
     test('single value doesnt match template', () => {
-      const modifiedOptions = extend({}, options, { str: 1 });
+      const modifiedOptions = assignDeep({}, options, { str: 1 });
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -247,7 +247,7 @@ describe('options validation', () => {
     });
 
     test('single enum value doesnt match template', () => {
-      const modifiedOptions = extend({}, options, { abc: 'testval' });
+      const modifiedOptions = assignDeep({}, options, { abc: 'testval' });
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -255,7 +255,7 @@ describe('options validation', () => {
     });
 
     test('multiple values dont match template', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         str: 1,
         abc: 'testval',
         nullbool: 'string',
@@ -269,7 +269,7 @@ describe('options validation', () => {
     });
 
     test('single nested value dont match template', () => {
-      const modifiedOptions = extend({}, options, { nested: { num: 'hi' } });
+      const modifiedOptions = assignDeep({}, options, { nested: { num: 'hi' } });
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -277,7 +277,7 @@ describe('options validation', () => {
     });
 
     test('single nested enum value dont match template', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: { abc: 'testabc' },
       });
       const result = validate(modifiedOptions, template);
@@ -287,7 +287,7 @@ describe('options validation', () => {
     });
 
     test('multiple nested values dont match template', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: { num: 'hi', abc: 'testabc' },
       });
       const result = validate(modifiedOptions, template);
@@ -298,7 +298,7 @@ describe('options validation', () => {
     });
 
     test('all nested values dont match template', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: { num: 'hi', abc: 'testabc', switch: 1 },
       });
       const result = validate(modifiedOptions, template);
@@ -308,7 +308,7 @@ describe('options validation', () => {
     });
 
     test('all nested values dont match template with foreign property', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: {
           foreign: 'foreign',
           num: 'hi',
@@ -323,7 +323,7 @@ describe('options validation', () => {
     });
 
     test('various values dont match template', () => {
-      const modifiedOptions = extend({}, options, {
+      const modifiedOptions = assignDeep({}, options, {
         nested: { switch: null },
         obj: 1,
         abc: 'testest',
@@ -343,7 +343,7 @@ describe('options validation', () => {
         foreignProp: 'foreign',
         foreignDeep: { a: 'A', b: 'B' },
       };
-      const modifiedOptions = extend(
+      const modifiedOptions = assignDeep(
         {},
         options,
         {
@@ -369,7 +369,7 @@ describe('options validation', () => {
     });
 
     test('nested object is string', () => {
-      const modifiedOptions = extend({}, options, { nested: 'string' });
+      const modifiedOptions = assignDeep({}, options, { nested: 'string' });
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -377,7 +377,7 @@ describe('options validation', () => {
     });
 
     test('nested object is null', () => {
-      const modifiedOptions = extend({}, options, { nested: null });
+      const modifiedOptions = assignDeep({}, options, { nested: null });
       const result = validate(modifiedOptions, template);
       const { validated } = result;
 
@@ -385,7 +385,7 @@ describe('options validation', () => {
     });
 
     test('nested object is undefined', () => {
-      const modifiedOptions = extend({}, options);
+      const modifiedOptions = assignDeep({}, options);
       modifiedOptions.nested = undefined;
       const result = validate(modifiedOptions, template);
       const { validated } = result;
@@ -409,7 +409,7 @@ describe('options validation', () => {
       const { warn } = console;
       console.warn = jest.fn();
 
-      const modifiedOptions = extend({}, options, { str: 1 });
+      const modifiedOptions = assignDeep({}, options, { str: 1 });
       validate(modifiedOptions, template, {}, false);
       expect(console.warn).not.toBeCalled();
 
@@ -421,15 +421,15 @@ describe('options validation', () => {
       console.warn = jest.fn();
 
       // str must be string
-      validate(extend({}, options, { str: 1 }), template, {}, true);
+      validate(assignDeep({}, options, { str: 1 }), template, {}, true);
       expect(console.warn).toBeCalledTimes(1);
 
       // abc must be A | B | C
-      validate(extend({}, options, { abc: 'some string' }), template, {}, true);
+      validate(assignDeep({}, options, { abc: 'some string' }), template, {}, true);
       expect(console.warn).toBeCalledTimes(2);
 
       // everthing OK
-      validate(extend({}, options, { abc: 'C' }), template, {}, true);
+      validate(assignDeep({}, options, { abc: 'C' }), template, {}, true);
       expect(console.warn).toBeCalledTimes(2);
 
       console.warn = warn;
diff --git a/packages/overlayscrollbars/tests/support/utils/object.test.ts b/packages/overlayscrollbars/tests/support/utils/object.test.ts
index e771c11..ab8ca83 100644
--- a/packages/overlayscrollbars/tests/support/utils/object.test.ts
+++ b/packages/overlayscrollbars/tests/support/utils/object.test.ts
@@ -1,9 +1,9 @@
-import { extend, keys, hasOwnProperty } from 'support/utils/object';
+import { assignDeep, keys, hasOwnProperty } from 'support/utils/object';
 import { isPlainObject } from 'support/utils/types';
 
 describe('object utilities', () => {
   // https://github.com/jquery/jquery/blob/master/test/unit/core.js#L965
-  describe('extend', () => {
+  describe('assignDeep', () => {
     // type DeepPartial<T> = T extends object ? { [K in keyof T]?: DeepPartial<T[K]> } : T
     type Deep = {
       foo?: {
@@ -30,11 +30,11 @@ describe('object utilities', () => {
       const optionsCopy: Settings = { xnumber2: 1, xstring2: 'x', xxx: 'newstring' };
       const merged: Settings = { xnumber1: 5, xnumber2: 1, xstring1: 'peter', xstring2: 'x', xxx: 'newstring' };
 
-      extend(settings, options);
+      assignDeep(settings, options);
       expect(settings).toEqual(merged);
       expect(options).toEqual(optionsCopy);
 
-      extend(settings, null, options);
+      assignDeep(settings, null, options);
       expect(settings).toEqual(merged);
       expect(options).toEqual(optionsCopy);
 
@@ -43,7 +43,7 @@ describe('object utilities', () => {
       const deep2copy: Deep = { foo: { baz: true }, foo2: document };
       const deepmerged: Deep = { foo: { bar: true, baz: true }, foo2: document };
 
-      extend(deep1, deep2);
+      assignDeep(deep1, deep2);
       expect(deep1.foo).toEqual(deepmerged.foo);
       expect(deep2.foo).toEqual(deep2copy.foo);
       expect(deep1.foo2).toBe(document);
@@ -51,21 +51,21 @@ describe('object utilities', () => {
       const arr = [1, 2, 3];
       const nestedArray: NestedArray = { arr };
 
-      expect(extend({}, nestedArray).arr).not.toBe(arr);
-      expect(Array.isArray(extend({ arr: {} }, nestedArray).arr)).toBeTruthy();
-      expect(Array.isArray(extend({ arr: {} }, nestedArray).arr)).toBeTruthy();
-      expect(isPlainObject(extend({ arr }, { arr: {} }).arr)).toBeTruthy();
+      expect(assignDeep({}, nestedArray).arr).not.toBe(arr);
+      expect(Array.isArray(assignDeep({ arr: {} }, nestedArray).arr)).toBeTruthy();
+      expect(Array.isArray(assignDeep({ arr: {} }, nestedArray).arr)).toBeTruthy();
+      expect(isPlainObject(assignDeep({ arr }, { arr: {} }).arr)).toBeTruthy();
 
       let empty: { foo?: any } = {};
       const optionsWithLength = { foo: { length: -1 } };
 
-      extend(empty, optionsWithLength);
+      assignDeep(empty, optionsWithLength);
       expect(empty.foo).toEqual(optionsWithLength.foo);
 
       empty = {};
       const optionsWithDate = { foo: { date: new Date() } };
 
-      extend(empty, optionsWithDate);
+      assignDeep(empty, optionsWithDate);
       expect(empty.foo).toEqual(optionsWithDate.foo);
 
       /** @constructor */
@@ -75,53 +75,57 @@ describe('object utilities', () => {
       const optionsWithCustomObject = { foo: { date: customObject } };
       empty = {};
 
-      extend(empty, optionsWithCustomObject);
+      assignDeep(empty, optionsWithCustomObject);
       expect(empty.foo && empty.foo.date === customObject).toBeTruthy();
 
       // Makes the class a little more realistic
       MyKlass.prototype = { someMethod() {} };
       empty = {};
 
-      extend(empty, optionsWithCustomObject);
+      assignDeep(empty, optionsWithCustomObject);
       expect(empty.foo && empty.foo.date === customObject).toBeTruthy();
 
       const MyNumber = Number;
 
-      let ret: any = extend({ foo: 4 }, { foo: new MyNumber(5) });
+      let ret: any = assignDeep({ foo: 4 }, { foo: new MyNumber(5) });
       expect(parseInt(ret.foo?.toString() as string, 10) === 5).toBeTruthy();
 
-      let nullUndef = extend({}, options, { xnumber2: null });
+      let nullUndef = assignDeep({}, options, { xnumber2: null });
       expect(nullUndef.xnumber2).toBe(null);
 
       // @ts-ignore
-      nullUndef = extend({}, options, { xnumber2: undefined });
+      nullUndef = assignDeep({}, options, {});
       expect(nullUndef.xnumber2).toBe(options.xnumber2);
 
       // @ts-ignore
-      nullUndef = extend({}, options, { xnumber0: null });
+      nullUndef = assignDeep({}, options, { xnumber2: undefined });
+      expect(nullUndef.xnumber2).toBe(undefined);
+
+      // @ts-ignore
+      nullUndef = assignDeep({}, options, { xnumber0: null });
       expect(nullUndef.xnumber0).toBe(null);
 
       const target = {};
       const recursive = { foo: target, bar: 5 };
 
-      extend(target, recursive);
+      assignDeep(target, recursive);
       expect(target).toEqual({ bar: 5 });
 
-      ret = extend({ foo: [] }, { foo: [0] });
+      ret = assignDeep({ foo: [] }, { foo: [0] });
       expect(ret.foo?.length).toBe(1);
 
-      ret = extend({ foo: '1,2,3' }, { foo: [1, 2, 3] });
+      ret = assignDeep({ foo: '1,2,3' }, { foo: [1, 2, 3] });
       expect(typeof ret.foo !== 'string').toBeTruthy();
 
-      ret = extend({ foo: 'bar' }, { foo: null });
+      ret = assignDeep({ foo: 'bar' }, { foo: null });
       expect(typeof ret.foo !== 'undefined').toBeTruthy();
 
       const obj = { foo: null };
-      extend(obj, { foo: 'notnull' });
+      assignDeep(obj, { foo: 'notnull' });
       expect(obj.foo).toBe('notnull');
 
       const func: { (): void; key?: string } = () => {};
-      extend(func, { key: 'value' });
+      assignDeep(func, { key: 'value' });
       expect(func.key).toBe('value');
 
       const defaults = { xnumber1: 5, xnumber2: 7, xstring1: 'peter', xstring2: 'pan' };
@@ -132,15 +136,15 @@ describe('object utilities', () => {
       const options2Copy = { xstring2: 'xx', xxx: 'newstringx' };
       const merged2 = { xnumber1: 5, xnumber2: 1, xstring1: 'peter', xstring2: 'xx', xxx: 'newstringx' };
 
-      settings = extend({}, defaults, options1, options2);
+      settings = assignDeep({}, defaults, options1, options2);
       expect(settings).toEqual(merged2);
       expect(defaults).toEqual(defaultsCopy);
       expect(options1).toEqual(options1Copy);
       expect(options2).toEqual(options2Copy);
 
-      expect(extend('', { foo: 1 })).toEqual({ foo: 1 });
-      expect(extend(null, { foo: null, deep: { foo: null } })).toEqual({ foo: null, deep: { foo: null } });
-      expect(extend(12, { foo: 1, deep: { foo: null, text: '' } })).toEqual({ foo: 1, deep: { foo: null, text: '' } });
+      expect(assignDeep('', { foo: 1 })).toEqual({ foo: 1 });
+      expect(assignDeep(null, { foo: null, deep: { foo: null } })).toEqual({ foo: null, deep: { foo: null } });
+      expect(assignDeep(12, { foo: 1, deep: { foo: null, text: '' } })).toEqual({ foo: 1, deep: { foo: null, text: '' } });
     });
   });