My users in App Script read their queue number from a Spreadsheet:
var array = sessionsSheets.getRange(row, 2, 1, 3).getValues()[0];
var questionNumber = array[2]; //This is the variable of interest, it is an integer signifying the queue
sessionsSheets.getRange(`D${row}`).setFormula(questionNumber+1); //This is the updated queue
They then update this queue number as seen above. This method works fine most of the time, but if you take two devices and run the script simultaneously, they both receive the same queue, and the script will halt for one of them because you need a unique queue number later on to use Slides API:
Slides.Presentations.batchUpdate({'requests': requests}, presentationId);
If promises worked, I would have simply put the Slides API line in a try block, and if an error of duplicity pops up, I would then call the same function recursively, up until overlap doesn't occur. However, promises are not supported in App Script, so what should I try instead?
I found a fix for my special case:
I used ScriptProperties in the Properties Services in App Script. When a user opens the application, he looks for the property of the key with his session's token. If it says busy, he waits and recursively tries the same function, if it says free, he changes it to busy, executes his APIs, then at the end changes it to free again.
Code Snippet:
function queueing(comment,name,token,sessionsSheets,row) {
var properties = PropertiesService.getScriptProperties();
var state = properties.getProperty(token);
if (state==null){
properties.setProperty(token , `busy@0`);
posting(comment,name,token,sessionsSheets,row,0);
} else if (state.includes("busy")){
Logger.log('Gotta wait my turn');
Utilities.sleep(2000);
queueing(comment,name,token,sessionsSheets,row);
} else if (state.includes("free")){
var questionNumber = state.split('@')[1];
properties.setProperty(token , `busy@${questionNumber}`);
posting(comment,name,token,sessionsSheets,row,questionNumber);
}}