HttpService.js

class HttpService {

    get(url) {

        return new Promise((resolve, reject) => {

            let xhr = new XMLHttpRequest();
            xhr.open('GET', url);
            xhr.onreadystatechange = () => {

                if (xhr.readyState == 4) {

                    if (xhr.status == 200) {

                        resolve(JSON.parse(xhr.responseText));
                    } else {

                        reject(xhr.responseText);
                    }
                }
            };
            xhr.send();
        });
    }

    post(url, dado) {

        return new Promise((resolve, reject) => {

            let xhr = new XMLHttpRequest();
            xhr.open("POST", url, true);
            xhr.setRequestHeader("Content-type", "application/json");
            xhr.setRequestHeader("Accept", "application/json")
            xhr.onreadystatechange = () => {

                if (xhr.readyState == 4) {

                    if (xhr.status == 200) {

                        resolve(JSON.parse(xhr.responseText));
                    } else {

                        reject(xhr.responseText);
                    }
                }
            };
            xhr.send(dado === undefined ? null : JSON.stringify(dado));
        });
    }
}