I have a simple test for $.ajax request, I get an error when I run this in my browser it says, "$. is undefined". can anyone help me. This is my function

 

function ajax(successCallback) {
	$.ajax({
		url: 'server.php',
		success: successCallback
	});
}
and my tests is
test('asynchronous test', function () { // Pause the test 
	stop();
	ajax(function () {
		ok(true)
	}) setTimeout(function () {
		start();
	}, 2000);
})

thanks