Vers requires an environment that supports the Promise/A+ specification as standardized in ES6. Node.js version 0.12.0 and up is great right out of the box (no --harmony flag necessary), as well as the latest versions of many browsers. To support older browsers, just include a Promise library such as Bluebird. Function options.getVersion: A function that accepts an object as its only argument, and returns either the current version identifier of the object as a number or string, or a Promise that resolves to the current version identifier. By default, Vers will use the object's version property if it exists, or 1 if it doesn't. number|string options.latest: The latest version identifier available for this model. If not specified, Vers will detect the latest version by calling Math.max on each version specified in addConverter(). For string-based versions, this option should be specified.