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

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

var result = a1.slice( 0, 2 ).concat( a2 ).concat( a1.slice( 2 ) );

document.write( result );