Excel users are used to create user defined functions (UDFs)—in Windows and Mac versions of the application. User defined functions can be simply written in Visual Basic Editor (VBE) using the Visual Basic for Applications (VBA) language.

User defined functions are not portable to Excel Online and Excel for iOS due to the missing VBA capability on these platforms. Therefore, Microsoft has improved the java-script API that was already being used for Office add-in development to allow developers to create custom functions attached to the add-ins. These functions, Excel Custom Functions, can be used like any other native or user defined functions in Excel.

The most beautiful aspect of this new functionality is that developers can take advantage of the power of widely used web services in Excel Custom Functions, and the end user doesn't even have to know about web application development.

This book starts with demonstrating Excel Custom Functions in Microsoft ScriptLab Office add-in, and shows the process of creating and debugging Excel Custom Functions by using Visual Studio Code and Node.js.