J2CL http request

J2CLHttp.java

import elemental2.dom.XMLHttpRequest;
import elemental2.promise.Promise;

public class Http {

    enum HttpMethod {
        GET, POST
    }

    public static Promise<String> request(HttpMethod method, String url) {
        return new Promise<>((resolve, reject) -> {
            XMLHttpRequest xhr = new XMLHttpRequest();
            xhr.onreadystatechange = e -> {
                if (xhr.readyState == XMLHttpRequest.DONE) {
                    resolve.onInvoke(xhr.responseText);
                }
                return null;
            };
            xhr.addEventListener("error",  /*NON-NLS*/ (e) -> {
                reject.onInvoke(xhr.statusText);
            });
            xhr.open(method.name(), url, true);
            xhr.send();
        });
    }
}

Promise<String> promise = Http.request(
    Http.HttpMethod.GET,
    "https://api.github.com"
);
promise.then(
    success -> {
        DomGlobal.alert("success: \n" + success);
        return null;
    },
    error -> {
        DomGlobal.alert("error: " + error);
        return null;
    }
);