ES6 Ajax Class

ajax.class.js

/* https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState */
const UNSENT            = 0,
      OPENED            = 1,
      HEADERS_RECEIVED  = 2,
      LOADING           = 3,
      DONE              = 4;

/* https://developer.mozilla.org/en-US/docs/Web/HTTP/Status */
const OK                = 200;

class ReadyStates {
    static get UNSENT() {
        return UNSENT;
    }

    static get OPENED() {
        return OPENED;
    }

    static get HEADERS_RECEIVED() {
        return HEADERS_RECEIVED;
    }

    static get LOADING() {
        return LOADING;
    }

    static get DONE() {
        return DONE;
    }
}

class HTTPStatus {
    static get OK() {
        return OK;
    }
}

export class Ajax {
    constructor() {
        this.xhr = new XMLHttpRequest();
    }

    onStateChangedEvent(xhr, onSuccess, onError) {
        return () => {
            if (xhr.readyState === ReadyStates.DONE && xhr.status === HTTPStatus.OK) {
                let contentType = xhr.getResponseHeader("Content-Type").toLowerCase();
                if (contentType === 'application/json') {
                    onSuccess(JSON.parse(xhr.responseText), contentType);
                } else {
                    onSuccess(xhr.responseText, contentType);
                }
            } else if (xhr.readyState === ReadyStates.DONE && xhr.status !== HTTPStatus.OK) {
                onError(xhr.responseText, xhr.status);
            }
        }
    }

    encodeData(data) {
        return Object.keys(data).map((key) => {
            return encodeURIComponent(key) + '=' + encodeURIComponent(data[key]);
        }).join('&');
    }

    get(endpoint, data, onSuccess, onError) {
        let xhr = this.xhr;
        xhr.open('GET', endpoint + '?' + this.encodeData(data));
        xhr.onreadystatechange = this.onStateChangedEvent(xhr, onSuccess, onError);
        xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
        xhr.send();
        return xhr;
    }

    post(endpoint, data, onSuccess, onError) {
        let xhr = this.xhr;
        xhr.open('POST', endpoint, true);
        xhr.onreadystatechange = this.onStateChangedEvent(xhr, onSuccess, onError);
        xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
        xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
        xhr.send(this.encodeData(data));
        return xhr;
    }
}