Ajax query function in native js using jquery syntax

ajax.js

function ajax ({                        // объект(!)
    url,                                // url
    type = 'GET',                       // *GET|POST
    data = {},                          // document.getElementById('formId') | FormData obj | serialized string | json | {}
    dataType = 'json',                  // output format: *'json' | 'html' | 'formData' (FormData obj, return json from the backend!)
    contentType = null,                 // Content-Type header string
    before = () => {},
    success = (response) => {},
    error = (response, status) => {},
    after = () => {}
}) {

    // before event
    if (typeof (before) == 'function') {
        before();
    }

    // data processing
    let formData;
    if (data instanceof FormData) {
        formData = data;
    } else {
        if (data.nodeName === 'FORM') {
            formData = new FormData(data);
        } else {
            formData = new FormData();
            if (typeof (data) == 'string') {
                try {
                    data = JSON.parse(data);
                } catch (e) {
                    data = unserialize(data);
                }
            }
            if (typeof (data) == 'object') {
                for ( let key in data ) {
                    formData.append(key, data[key]);
                }
            }
        }
    }

    // worker
    let response = null;
    let request = new XMLHttpRequest();

    request.onreadystatechange = function () {
        if (this.readyState === 4) {
            if (this.status === 200) {

                try {

                    // success event
                    response = this.response;
                    if (typeof (success) == 'function') {
                        switch (dataType) {
                            case 'json':
                                response = JSON.parse(this.response);
                                break;
                            case 'formData':
                                let formResponse = JSON.parse(this.response);
                                response = new FormData();
                                for ( let key in formResponse ) {
                                    response.append(key, formResponse[key]);
                                }
                                break;
                            default:
                        }

                        success(response);
                    }

                } catch (e) {
                    response = null;
                    console.error('AJAX():', e);
                }

            } else {

                // error event
                console.error('AJAX(): ' + this.status + ' ' + this.statusText);
                if (typeof (error) == 'function') {
                    error(this.response, this.status);
                }

            }

            // after event
            if (typeof (after) == 'function') {
                after();
            }

        }
    };

    // send
    request.open(type, url);
    request.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
    if (contentType) {
        request.setRequestHeader('Content-Type', contentType);
    }
    request.send(formData);
}

function unserialize(s) {
    a = s.split('&');
    out = {};
    for (let k in a) {
        let b = a[k].split('=');
        out[b[0]] = decodeURIComponent(b[1]) ?? '';
    }
    return out;
}