https://dev.office.com/blogs/azure-machine-learning-javascript-custom-functions-and-power-bi-custom-visuals-further-expand-developers-capabilities-with-excel

Today at Build, we announced a powerful set of capabilities to the Excel platform, offering developers, data scientists, and power users more opportunities to better work with data. Developers and data scientists can now to execute custom functions locally in JavaScript or with Microsoft Azure Machine Learning services to create their own powerful additions to Excel's catalog of formulas. Excel has enabled an out of the box bridge between business users and data science teams, to accelerate the digital transformation of businesses. With new support for Power BI Custom Visuals, developers will be able to build new kinds of data visualizations in Excel — giving users a much more diverse charting experience, right in Excel. Finally, Excel will integrate with Flow, allowing users sending data directly from a spreadsheet to a range of services. Read on to learn more.

Machine Learning and JavaScript custom functions for developers and data scientists.

Custom functions (similar to "user-defined functions" or "UDFs") are ways for developers to extend Excel's built-in set of formula functions. Users can create their own functions locally in JavaScript or with Machine Learning teams earning services. They can see and run these functions in Excel alongside built-in functions like =SUM or =VLOOKUP. The best part is that those same functions will work everywhere add-ins do: on PC, Mac, and iPad, and in Excel Online.