Excel already has the ability to write custom functions in Javascript (and VBA), this is the ability to write them in the formula language rather than JavaScrip

Excel already has the ability to write custom functions in Javascript (and VBA), this is the ability to write them in the formula language rather than JavaScript. (So no, Google Sheets does not support this) The latest improvements in Excel really do seem to be re-widening the gap between Google Sheets and Excel (Dynamic array formulas, Let, Custom data types, powerquery improvements...)