JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">

JavaScript

Array.prototype.injectArray = function( idx, arr ) {
    return this.slice( 0, idx ).concat( arr ).concat( this.slice( idx) );
};

var a1 = [1,2,3,4,5];
var a2 = [21,22];

var result = a1.injectArray( 2, a2 );;

document.write( result );