JScript arrays don't seem to support the indexOf method like JavaScript arrays do.
See e.g. https://stackoverflow.com/questions/26073853/js-file-doesnt-work.
The suggestion there is to roll your own function:
function arrayContains(array, value){
for (var i = 0, e = array.length; i < e; ++i) {
if (array[i] === value) {
return true;
}
}
return false;
}