JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.spliceArray = function(index, insertedArray) {
  var postArray = this.splice(index);
  inPlacePush(this, insertedArray);
  inPlacePush(this, postArray);

  function inPlacePush(targetArray, pushedArray) {
    // Not using forEach for browser compatability
    var pushedArrayLength = pushedArray.length;
    for (var index = 0; index < pushedArrayLength; index++) {
      targetArray.push(pushedArray[index]);
    }
  }
}
var arr1 = [];
var arr2 = [];

var maxLength1 = 1205967;
var maxLength2 = 1300000; // YAY!

for (var i = 0; i < maxLength2; i++) {
  arr1.push(i);
  arr2.push(i);
}

arr1.spliceArray(3, arr2);
console.log(arr1);