What does console.log(9007199254740992 + 1) output?

Console output

Why

The value 9007199254740992 is 2^53, exactly one past MAX_SAFE_INTEGER. At this magnitude doubles can only represent even integers, so 2^53 + 1 (an odd number) is not representable and rounds back to 2^53. The addition silently has no effect. That is a classic source of off-by-one bugs with large IDs.