The good news is that formulajs had a huge number of those functions implemented in JS already. Almost all of the time was spent on the engine, which wasn't a huge amount of code.
The problem with porting the code to JS is that a.) nothing is named, b.) there's no real way to organize the code you've written because you're going from a spatial way of organizing code to imperative script, and c.) the actual design of the spreadsheet wasn't known to any engineers (it was designed by a data scientist, or perhaps an analyst). The work of translating would have meant really understanding what the thing is so that it can be turned into functions and modules. It also would have still required getting Excel function equivalents, since there's not a 1:1 equivalence between Excel and what's available in the JS standard lib.
The problem with porting the code to JS is that a.) nothing is named, b.) there's no real way to organize the code you've written because you're going from a spatial way of organizing code to imperative script, and c.) the actual design of the spreadsheet wasn't known to any engineers (it was designed by a data scientist, or perhaps an analyst). The work of translating would have meant really understanding what the thing is so that it can be turned into functions and modules. It also would have still required getting Excel function equivalents, since there's not a 1:1 equivalence between Excel and what's available in the JS standard lib.