Redirect to 400 on xmlhttprequest

I am posting using xmlhttprequest as follows

// xhr response handler
xhr.onreadystatechange = function () {
    if (xhr.status == 200 && xhr.readyState == 4) {


        handleSuccess(xhr.responseText);

    } else if (xhr.status == 400) {
        console.log("400000000000000000000");
        // alert("Unable to process data");
        // window.location.reload();
        sendRequestSimple("400.html");
        // fetch("/400")
        //     .then(function() {
        //         console.log("ok");
        //     }).catch(function() {
        //     console.log("error");
        // });
    }
    else {
        console.trace("status: " + xhr.status);
        console.trace("readystate: " + xhr.readyState);
    }
};

// Send the Data.
xhr.send(formData);

I am unable to get it to redirect to 400 error page in an error scenario. What is the correct way? As coded above..it will not redirect.

Many thanks in advance