    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.
    }