ion::NewInitArray calls NewDenseAllocatedArray, but js_Array calls NewDenseUnallocatedArray. To match the interpreter and JM, we should allocate space for array literals but not for "new Array(x)". Consider also this micro-benchmark: -- function f() { for (var i=0; i<1000000; i++) var a = Array(100); return a; } var t = new Date; f(); print(new Date - t); -- JM+TI: 18 ms Ion: 477 ms