Get an array of Permission Levels (Role Definitions) for the current site using REST

GetSharePointPermissionLevels.js

// This script returns an array of Role Definition objects, containing the Permission Levels for the site

// See http://msdn.microsoft.com/en-us/library/office/jj244931(v=office.15).aspx for examples of how to use the
// SP.PermissionKind object to assign permissions to a Role Definition

// replacement function for console.log(), which avoids 'undefined' exception in IE 8
window.LogMsg = function (msg) {
    if (window.console) {
        console.log(msg);
    }
};

// ensure library is loaded before executing a function
// ns = namespace (i.e. jQuery)
// url = url of library (i.e. from a CDN)
// func = a function to execute when the library has loaded
window.EnsureLibrary = function (ns, url, func) {
	// execute a function when a library has loaded
	var executeOnLoaded = function (ns, func) {
		if (!window[ns]) {
			setTimeout(function () {
				executeOnLoaded(ns, func);
			}, 100);
		} else {
			LogMsg(ns + " loaded");
			func();
		}
	};
 
	if (!window[ns]) {
		var script = document.createElement("script");
		script.setAttribute("src", url);
		var head = document.getElementsByTagName("head")[0];
		head.appendChild(script);
	}
 
	executeOnLoaded(ns, func);
};

(function() {
	var jQueryURL = "//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js";
	var restEndpoint = _spPageContextInfo.webAbsoluteUrl + "/_api/web/RoleDefinitions?$select=Name,Description,Id,BasePermissions";
	
	EnsureLibrary("jQuery", jQueryURL, function() {
		jQuery.noConflict();
		jQuery.ajax({
			url: restEndpoint,
			cache: false,
			async: true,
			dataType: "json"
		}).done(function (data) {
			var data = data.value;
			LogMsg(data);
		});
	});
})();