OK, I have heard some weird news from one company.

They gave their staffs 3 months notice but still cannot hire somebody to do.

Then from their Exit Audit checklist, suddenly bosses can ask them to do something which is not required now but want it done so that the new person comes in, don't need to rush.

I wonder right, if the person really cannot finish the ad hoc items on time, does the company have a right to keep the staff and not allow him/her to leave?

Not allow to leave? I believe company can ask who want to extend with extra pay but cannot stop employee if they want to leave.

Resignation letter is just a notice, not to ask for approval. Company got no right not to let employee to leave

Strictly speaking, no. Letter say 30 day notice, even if staff purposely never handover, technically employer can't do anything about it.

More like for court of public opinion. If it's clear that the ad hoc work is given with intention to block the staff from leaving, doesn't look good on the employer as well.

Thanks for the reply guys. So meaning, if the employer decided to hold the leaving staff's salary until the leaving staff submits all the work, the leaving staff can do legal method?

just report them to your nearest labour office, save all communications as evidence

they will mediate, give u advise and/or refer them to IR courts if needed