Latest favourite snippet

isFirstOnly = undefined;

if ($.inArray(isFirstOnly,[0,1]) ){
    alert('Yep')
}
​
// -> alert pops up because inArray false is "-1". Yeeey, delicate!

One solution:

isFirstOnly = undefined;

if ($.inArray(isFirstOnly,[0,1])!=-1 ){
    alert('Yep')
}