New Google Sheets custom functions sometimes display “Loading…” indefinitely

Question

SPECIFIC FOR: \"NEW\" google sheets only.

This is a known issue as highlighted by google in the new sheets.

Issues: If you write complex^* custom f

Answer 1

One cause: Permissions needing authorizing.

As far as {this problem, better phrased the cell result(s) of a custom function displaying the disgustingly-vague message ‘Loading... Error: loading data...’}, indeed in the case where all instances of the same/similar custom function call displaying this error, is that Google Sheets needs permissions to run the script (often additionally: meaning in the past it didn't need these), so instead of {acting appropriately: then prompting the user for these permissions else returning that error}, Sheets instead hangs with this disgustingly vague error.

Additional permissions can be needed from 1 or more:

1. Google App Scripts has since rewriting their permission structure --how this problem now just happened to me, per my internal note O80U3Z.
2. Your code or some library it uses made changes to require more access ...but in this case you have a much better chance of guessing the cause of this disgustingly-vague error, so hopefully won't be reading here.

To fix, I explicitly ran my GAS spreadsheet code by both: clicking one of my custom menu functions and, in the ‘script editor’, running one of my custom JS functions notably the ‘onOpen()’ since that is most comprehensive. The first promoted me for indeed new permissions, via popup ‘Authorization Required
The application "MM6ZBT(MM6Z83 script)" needs authorization to run.’, though onOpen() also did this in cases of GAS revising its permissions since we used that sheet. Then, as I was still getting this ‘Loading...’ error, I reloaded the web page (so the sheet), and, at least for these cases of this disgustingly vague error, it was gone and the computations worked fine :-)