Javascript dynamic parameter identification

I wrote it like this:
Function foo (x ){
If (arguments [1]) {
// Do something ..
} Else {
// Do other ..
}
}
However, no matter how many parameters are passed in, the if (arguments [1]) step is skipped. When I was about to go crazy, I finally succeeded.
Function foo (x ){
If (arguments [1]! = Undefined ){
// Do something ..
} Else {
// Do other ..
}
}
I think of it in the Javascript authoritative guide that null and undefined are sometimes equal, but sometimes they are unequal. It is estimated that this is the case.