function f(arr, b) {
var res = "";
var a;
if (b)
a = arr;
for (var i=100; i>-200; i--) {
if (i in a) {
res += i;
}
}
return res;
}
assertEq(f([1, , 2, 3], true), "320");
try {
f([1, , 2, 3], false);
assertEq(0, 1);
} catch(e) {
assertEq(e instanceof TypeError, true);
}