WAL Replication query

I am  trying  WAL replication,the setup is working  with a primary and Standby server.

Query is  :   When the primary crashes  due an unforeseen reason,  what may happen to the uncommitted transactions at primary ?  

Those  won't replicate to the standby right ?

How much data loss occurs in this case ?  

What is the mechanism to overcome this uncommitted transaction loss due to primary server crash which is not   replicated to the standby  server ?

Thank you,

 Krishane