How to pass errors conditions - google-apps-script

Question

In web app development I would like a consistent way to catch and report error conditions. For example, a database update routine may detect a variety of error conditions and ideally I would like the application to capture them and report gracefully. The code below din't work because retdiag is undefined when error is thrown...
function saveData(app,e) {
var db ;
var retdiag = "";
var lock = LockService.getPublicLock();
lock.waitLock(30000);
try {
// e.parameters has all the data fields from form
// re obtain the data to be updated
db = databaseLib.getDb();
var result = db.query({table: 'serviceUser',"idx":e.parameter.id});
if (result.getSize() !== 1) {
throw ("DB error - service user " + e.parameter.id);
}
//should be one & only one
retdiag = 'Save Data Finished Ok';
}
catch (ex) {
retdiag= ex.message // undefined!
}
finally {
lock.releaseLock();
return retdiag;
}
}
Is there a good or best practice for this is GAS?

Answer 1

To have a full error object, with message and stacktrace you have to build one, and not just throw a string. e.g. throw new Error("DB error ...");
Now, a more "consistent" way I usually implement is to wrap all my client-side calls into a function that will treat any errors for me. e.g.
function wrapper_(f,args) {
try {
return f.apply(this,args);
} catch(err) {
;//log error here or send an email to yourself, etc
throw err.message || err; //re-throw the message, so the client-side knows an error happend
}
}
//real client side called functions get wrapped like this (just examples)
function fileSelected(file,type) { return wrapper_(fileSelected_,[file,type]); }
function loadSettings(id) { return wrapper_(loadSettings_,[id]); }
function fileSelected_(file,type) {
; //do your thing
}
function loadSettings_(id) {
; //just examples
throw new Error("DB error ...");
}