A Viewer is an HTML file with embedded JavaScript that has access to some user-specified files. When a Viewer is opened, a dialogue first appears that allows you to choose a set of files in your projects, for which download URLs are then generated. These files' download URLs are then made available to the Viewer through a snippet of JavaScript code. From there, a Viewer can link to another web server, such as the UCSC genome browser, or run some JavaScript within the Viewer itself -- for instance, by using an outside plotting library such as IGV.js.