Universal Ajax Form handler by vanilla JavaScript

example.js

FormAjax('#loginform', 'sb/v1/user/signin');

formajax.js

/**
 * from https://gist.github.com/uptimizt/34ce8e582e256eb2c3c3b612b23188a0
 */
var FormAjax = function(elementSelector, ep, args = []){

    if (document.querySelector(elementSelector)) {
        var form = document.querySelector(elementSelector);
    } else {
        return false;
    }

    var restApiEndpoint = wpApiSettings.root + ep;

    form.addEventListener('submit', function(event) {
        event.preventDefault();

        var formData = new FormData(form);
        var submitBtnText = '';
        var message = '';
        var submitBtn = form.querySelector('input[type="submit"]');
        var request = new XMLHttpRequest();

        // submitBtn.disabled = true;
        submitBtnText = submitBtn.value;
        submitBtn.value = 'Подождите...';

        request.open("POST", restApiEndpoint);
        request.setRequestHeader('X-WP-Nonce', wpApiSettings.nonce);
        request.send(formData);

        request.onload = function() {

        var message = '';

        if (request.status == 200) { // analyze HTTP status of the response

            var response = JSON.parse(request.response);

            if ( args.successCallback && typeof args.successCallback === 'function' ) {
                args.successCallback(response);
                return;
            }
            
            if(response.success){
            message = '<p class="success">' + response.data.message + '</p>';
            } {
            message = '<div class="error_login"><p class="error">' + response.data.message + '</p></div>';
            }

            // console.log(response);

            form.querySelector('.response').innerHTML = message;

            if (response.data.redirect) {
                window.location.href = response.data.redirect;
            }

            submitBtn.disabled = false;
            submitBtn.value = submitBtnText;

        } else {
            form.querySelector('.response').innerHTML = '<div class="error_login"><p class="error">' +
                    'Что-то пошло не так. Пожалуйста попробуйте еще раз.</p></div>';
            // Снимаем ограничение с кнопки и возврошаем прежнее зночение
            submitBtn.disabled = false;
            submitBtn.value = submitBtnText;
        }

        };

        request.onerror = function() {
        submitBtn.disabled = false;
        submitBtn.value = submitBtnText;
        };
    });
};