javascript Inserting values in between an array

Array.prototype.insert = function ( index, item ) {
    this.splice( index, 0, item );
};myArray.splice(index, itemsToDelete, item1ToAdd, item2ToAdd, ...)
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;

function myFunction() {
  fruits.splice(2, 0, "Lemon", "Kiwi");
  document.getElementById("demo").innerHTML = fruits;
}