Custom XHR wrapper with JSON parser

XHR.ts

import {Promise} from "es6-promise";

export class Header {
	header: string;
	data: string;
	constructor(header: string, data: string) {
		this.header = header;
		this.data = data;
	}
}

export class Data {
	headers: string;
	body: string;
	text: string;
	json: any;
	type: string;
	statusCode: number;
	statusText: string;
	
	public static FromXHR(jsXHR: XMLHttpRequest): Data {
		var data = new Data();
		data.headers = jsXHR.getAllResponseHeaders();
		data.body = jsXHR.responseBody;
		data.text = jsXHR.responseText;
		
		if (/Content-Type: application\/json/i.test(data.headers)) {
			data.json = JSON.parse(data.text);
		}
		
		data.type = jsXHR.responseType;
		data.statusCode = jsXHR.status;
		data.statusText = jsXHR.statusText;
		return data;
	}
}

export class XHR {
	
	private sendCommand(method: string, url: string, headers: Array<Header>, data: string = ""): Promise<Data> {
		return new Promise<Data>((resolve, reject) => {
			var jsXHR = new XMLHttpRequest();
			jsXHR.open(method, url);
		
			if (headers != null) {
				headers.forEach((header) => {
					jsXHR.setRequestHeader(header.header, header.data);
				});
			}
		
			jsXHR.onload = (ev) => {
				if (jsXHR.status < 200 || jsXHR.status >= 300) {
					reject(Data.FromXHR(jsXHR));
				}
				resolve(Data.FromXHR(jsXHR));
			}
			
			jsXHR.onerror = (ev) => {
				reject('Error ' + method.toUpperCase() + 'ing data to url "' + url + '", check that it exists and is accessible');
			};
		
			if (method == 'POST') {
				jsXHR.send(data);
			} else {
				jsXHR.send();
			}
		});
	}
	
	public get(url: string, headers: Array<Header> = null): Promise<Data> {
		return this.sendCommand('GET', url, headers);
	}
	
	public post(url: string, data: string = "", headers: Array<Header> = null): Promise<Data> {
		return this.sendCommand('POST', url, headers, data);
	}
}