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 functions in google-apps-script for google sheets, you will occasionally run into cells which display a red error box around the cell with the text "Loading..."

Google has suggested:

If this occurs, try reloading the page or renaming the function and changing all references to the new name.

However for other developers experiencing this issue (and who are unable to escape the "loading..." error), I've written my findings in the answer below on how to get past this (with limitations) consistently.

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^*We're treating this question as the canonical answer for Google Sheet's indefinite "Error... Loading data" problem. It's not limited to complex or slow functions.

Answer 1

I also had the infinite loading issue with the following function.

// check if an item can be checked off
function checkedOff( need, have ) {
  var retStr = "nope";
  if( have >= need ){
    retStr = "yep";
  }
  return retStr;
};

Turns out you shouldn't have a trailing ";". Removing the semicolon solved the problem.

// check if an item can be checked off
function checkedOff( need, have ) {
  var retStr = "nope";
  if( have >= need ){
    retStr = "yep";
  }
  return retStr;
}

This runs as one would expect.