JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = new customArray();
alert("arr.length initially returns " + arr.length);

arr.insert(10,"stuff"); // equivalent to arr[10] = "stuff"
var val = arr.get(10); // equivalent to var val = arr[10]

alert("Stored value of '" + val + "' in index " + "10");
alert("arr.length now returns " + arr.length);

function customArray(){
    this.length = 0;
    this.insert = function(index, value){
        this[index.toString()] = value;
        if (index > this.length){
             this.length = index;   
        }
    }
    this.get = function(index){
        return this[index.toString()];
    }
}