Excel users are used to build custom functions—called user-defined functions (UDFs)—in Windows and Mac versions of the application. UDFs can be simply written in Visual Basic Editor (VBE) using the Visual Basic for Applications (VBA) language. UDFs are not portable to Excel Online and Excel for iOS due to the missing VBA capability on these platforms. Therefore, Microsoft has improved the JavaScript API that was already being used for Office add-in development to allow developers to create custom functions attached to the add-ins. These custom functions can be used like any other native functions or UDFs in Excel. While Microsoft calls these functions JavaScript custom functions, they can also be called JavaScript UDFs, which is the name used in this book. This book shows the process of creating JavaScript UDFs in Excel in Developer Preview for Office Insider program subscribers.