Javascript AJAX call

ajax.js

ajax = (url, data, success, error, method) => {
	var params = typeof data == 'string' ? data : Object.keys(data).map(
			k => encodeURIComponent(k) + '=' + encodeURIComponent(data[k])
		).join('&');

	if(method === undefined)
		method = "POST";

	var xhr = new XMLHttpRequest();
	xhr.open(method, url);
	xhr.onreadystatechange = e => {
		if (xhr.readyState > 3) {
			if(xhr.status == 200) {
				if(success != undefined) {
					try {
						success(JSON.parse(xhr.responseText));
					} catch(e) {
						success(xhr.responseText);
					}
				}
			} else {
				error != undefined ? error() : 0;
			}
		}
	};
	xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	xhr.send(params);
	return xhr;
};