Truthy/Falsy behaviour in ServiceNow Javascript

I had a strange issue the other day - I was reviewing some Javascript code in a Script Include I had written some time before. I was convinced it should have a bug, but it passed extensive testing with flying colours. The code was something like:

var error_array = getListOfErrorsOrReturnEmptyArray();

if (error_array) {
    alertErrorCodes(error_array);

} else { // no errors
    everythingIsAllGood();
}

The code was expecting an empty array to be treated as false. (ie. 'falsey' ) - I had written it shortly after doing some Python code. In Python an empty array is falsy. but in Javascript an empty array should be truthy:

"In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy. That is, all values are truthy except false, 0, -0, 0n, "", null, undefined, NaN, and document.all."

The code (in a pure Javascript perspective) should read something like :

if (error_array && error_array.length) {

    alertErrorCodes(error_array);

} else { // no errors
    ...
}

So why did the original code work and treat an empty array as false? It was running as a server-side script on ServiceNow. It looks like ServiceNow have modified something so that empty arrays are treated at false. This is actually nicer in my opinion (Python does the same) - but it’s not standard Javascript.

This also reminded me of the way boolean glideElements also get the special truthy/falsy treatment: E.g.

if (incident_gr.active) doit();

If incident_gr.active were a normal JavaScript object in a normal JavaScript run-time this would always be truthy, but in the ServiceNow ServerSide environment, active is both a GlideElement object but can be falsy if the element value is falsy.

This is a perfectly acceptable (and somewhat encouraged) ServiceNow construct but would lead to errors outside of the ServiceNow ecosystem (even possibly in client-side scripts running in the browsers Javascript Run-Time). So it's not a portable construct if you plan to leverage JavaScript outside of ServiceNow..