Javascript's Number.isSafeInteger(x) yields false if abs(x) > 54

The Javascript function Number.isSafeInteger(val) yields false if abs(val) > 54.

The implementation only looks correct at the first glance .

NumberPrototype::method_isSafeInteger(...) {

    return Encode(dv == iv && std::fabs(iv) <= (2^53)-1);

}