Mastering JavaScript Functional Programming

Detecting Ajax

Let's go back a bit to the times when Ajax started to appear. Given those different browsers implemented Ajax calls in distinct fashions, you would always have to code around those differences:

function getAjax() {    let ajax = null;    if (window.XMLHttpRequest) {        // modern browser? use XMLHttpRequest        ajax = new XMLHttpRequest();            } else if (window.ActiveXObject) {        // otherwise, use ActiveX for IE5 and IE6        ajax = new ActiveXObject("Microsoft.XMLHTTP");            } else {        throw new Error("No Ajax support!");    }        return ajax;}

This worked but implied that you would re-do the Ajax check for each and every call -- even though the results of the test wouldn't ever change. There's a more efficient way to do so, and it has to do with using functions ...