test.cpp

unsigned start = 0;
if (value.isUInt32())
    start = value.asUInt32();
else {
    double valueAsNumber = value.toInteger(exec);
    if (exec->hadException())
        return jsUndefined();
    if (valueAsNumber < 0)
        start = 0;
    else {
        if (valueAsNumber > length)
            start = length;
        else
            start = static_cast<unsigned>(valueAsNumber);
    }
}