Send XMLHttpRequest requests

xhr.js

function xhr(method, hdrs, action, data, callback) {
	var r = new XMLHttpRequest();

	r.onreadystatechange = function() {
		if(r.readyState == 4 && r.status == 200)
			callback(JSON.parse(r.responseText)); /* always expect JSON */
	};

	switch(method.toUpperCase()) {
		case "POST":
			r.open(method, action, true);
			if(data && !(data instanceof FormData)) {
				r.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
				data = serialize(data);
			}
			break;
		case "GET":
			if(data) {
				data = serialize(data);
				action += ((action.indexOf('?') == -1) ? '?' : '&')+data;
				data = null;
			}
			r.open(method, action, true);
			break;
		default:
			console.error(method+": method not supported");
			return;
	}
	if(hdrs)
		for(var h in hdrs)
			r.setRequestHeader(h, hdrs[h]);
	r.send(data);
}