What exactly is a JavaScript Pop up Window generator? A JavaScript pop up window generator or pop up maker is a computer application online advertisers use to design, develop a piece of code to install and maintain what is known as a pop up advertisement or pop up window on one or many websites. These applications like most applications range from the very basic to the more complex ones which usually come with a lot more features. The simple applications design and develop very basic pop up ads that are block able while the complex applications have many added features like hover ads or the ability to add images, audio, or video.