The following cfml returns undefined
when using ACF 2018 or 2021, but throws an error when using Lucee 5.3.7.47 using TryCF.com. (I was testing this to determine if it would return the first “defined” item in the array versus simply array[1].)
<cfscript>
a = [];
arrayResize(a, 15);
a[15]=1;
writeoutput("len = #arrayLen(a)#<br>");
writedump(arrayfirst(a));
</cfscript>