Checking Whether A String Is All Digits

I'm trying to check whether a string is all digits. This part is
easy:

function allDigits( str ) {
var foo=str.split( '' ); // better than charAt()?
for( var idx=0; idx < foo.length; idx++ ) {
if( !isDigit(foo[idx]) ) {
return false;
}
}
return true;
}

I'm not sure about how to implement isDigit(). Is this the best way?

function isDigit( s ) {
if( s.length > 1 ) {
return false;
}
var nums=&#65471;?'
return nums.indexOf(s) != -1;
}