WD.js is a JavaScript library that allows you to write tests in parallel across multiple browsers using Selenium and provides a more efficient way to test your code base. The WD.js library comes in three flavors: Parallel WD, Synchronous Parallel WD, and wd-unit. These libraries address the pain of heterogeneity of platforms on which your code will run by letting you declare the browsers in which you want the test run and then running it in parallel while keeping it close to vanilla browser code. The Parallel WD library is based on asynchronous callbacks, while the Synchronous Parallel WD library follows classical procedural principles. The wd-unit library automates the running of your JavaScript unit tests using Selenium, allowing you to run everything in the cloud with a service like Sauce Labs across multiple browsers in parallel. With these libraries, you can write your tests once and have them run in parallel, reducing the time it takes to test your code base.