Synchronous file load
From JSXGraph Wiki
var brd = JXG.JSXGraph.loadBoardFromFile('jxgbox', 'http://jsxgraph.uni-bayreuth.de/geonext/Unendlich.gxt', 'Geonext');
var brd = JXG.JSXGraph.loadBoardFromFile('jxgbox', 'http://jsxgraph.uni-bayreuth.de/geonext/Unendlich.gxt', 'Geonext');