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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); }