env.addTest(name, fn)

Add a custom feature-detection test where name is a string which uniquely identifies your feature test and fn is a function that returns true if the browser supports the feature, and false if it does not.

joint.env.addTest('customTest', function() {
	// Just as an example, we will always return true here.
	return true;
});

if (joint.env.test('customTest')) {
    // Feature is supported.
}