H1b extensn with company A pending. . H1b change of employer with Company B approved

H1b expires 31st december , H1b extension with company A pending. . H1b change of employer with Company B approved .
With this scenario can I join employer B after 31st december as it is already approved. Reason to wait is - still not recieved approval notice in mail and then 2 weeks of notice period .

Yes you can join empB even if your EOS for empA is pending.

you meant I can join company B after current I94 expiry i.e. after 31st december right ?

Sorry missed the question. So in your case you have a pending EOS with employer A which can act as a bridge application if you join employer B after your current I-94 expires however as you will be resigning before 31st December, chances are that your employer may already submit request to USCIS to withdraw your EOS effective 31st December. If that happens, there will be no bridge application and you will be out of status after 31st December.
I highly recommend to join the new employer before your current I-94 expires.
Talk to the immigration lawyer of your new employer to discuss your situation and act per their advice.

so even if the employer B petition is approved , still there will be bridge petition ?

When a change of employer (aka transfer) petition is approved with expected start date, USCIS expects you to join the new employer while your current I-94 is still valid and you are actively employed.

In case of further questions, please consult your employer B immigration lawyer as they understand your case better.