[[Google Apps Script]] allows you to run both client-side and server-side code. The client-side code is all the code that manipulates the user interface. My practice is to store all of this code in a `JavaScript.html` file. The server-side code interacts with Google Sheets, Docs or other data. This is stored in the `Code.gs` file. For example, if you want to read data from a spreadsheet, you would write a function in `Code.gs`. If you want to display that data in a web application, you would write a function in the `JavaScript.html` file. To pass the data between the two, use a success handler.