How Do I Write a Custom Asserter For a WD waitFor?

Does anyone know whow to write a custom asserter for a WD waitFor statement. WD's github provides a command file here:

http://ift.tt/2lrIZSX

I am working with this method:

waitForElementByXPath(value, asserter, timeout, pollFreq, cb) -> cb(err, el)

the comments also tell us:

* asserter like: function(browser , cb) -> cb(err, satisfied, return_value)

What I need is a waitFor that takes an xpath and then waits for the element isDisplayed to be True or False. Is there a full fledged javascript api for wd anywhere?