Boilerplate for Cross-browser Scripting in Vanilla JavaScript

ajax.js

function ie9 () {
    let div = document.createElement("div");
    div.innerHTML = "<!--[if lte IE 9]><i></i><![endif]-->";
    return div.getElementsByTagName("i").length == 1;
}

function getXHR() {
    if (ie9()) {
        try {
            return new ActiveXObject('Microsoft.XMLHTTP');
        } catch { }

        try {
            return new ActiveXObject('Msxml2.XMLHTTP.6.0');
        } catch { }

        try {
            return new ActiveXObject('Msxml2.XMLHTTP.3.0');
        } catch { }

        try {
            return new ActiveXObject('Msxml2.XMLHTTP');
        } catch { }

        return null;
    }

    return new XMLHttpRequest();
}

let xhr = getXHR();

if (!xhr) {
    /* You should always check whether `xhr` object is null.
       Write some code to handle such condition properly. */
    // return;
}

xhr.onreadystatechange = function () {
    if (4 === xhr.readyState) {
        if (200 === xhr.status) {
            /* Implement your code here. */
        }
        else {
            /* Any HTTP status other than 200. */
        }
    }
}

if (!ie9()) {
    xhr.onerror = function () {
        /* Write code to handle error condition.
           The event is only valid on IE 10 or above. */
    };
}

/* Modify the code here to fit your own condition. */
xhr.open("POST", `https://example.com/path/to/post/`);

/* JSON is the de facto standard format for Ajax events.
   You may not need to modify anything here. */
xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");

/* Modify the code here to fit your own condition. */
let data = "";

if (ie9()) {
    setTimeout(function () {
        xhr.send(JSON.stringify({ data: data }));
    }, 0);
}
else {
    xhr.send(JSON.stringify({ data: data }));
}