JavaScript Array prototype Constructor

The JavaScript Array prototype constructor is for adding new methods and properties to array object. These properties and methods will be available for each array.

Following is the code for the array prototype constructor −


 Live Demo

<!DOCTYPE html>
<html lang="en">
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
   body {
      font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
   .sample {
      font-size: 20px;
      font-weight: 500;
<h1>JavaScript Array prototype Constructor</h1>
<div class="sample"></div>
<button class="Btn">CLICK HERE</button>
Click on the above button to convert the string to upper and lowercase
   Array.prototype.upperLower = function () {
      for (let i = 0; i < this.length; i++) {
         if (i % 2 == 0) this[i] = this[i].toUpperCase();
         else this[i] = this[i].toLowerCase();
   let fillEle = document.querySelector(".sample");
   let arr = ["H", "E", "L", "L", "O"];
   fillEle.innerHTML = arr;
   document.querySelector(".Btn").addEventListener("click", () => {
      fillEle.innerHTML = arr;


On clicking the “CLICK HERE” button the custom array function will be called −