Redo Log Files

 I know that :-

The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.

I want to ask , how redo log files used to recover data when an unexpected power outage occurs and terminates database operation,

of course oracle will not able to write the data in memory to datafiles and the data will lost, 

(I think that oracle also will be late to write the data to the redo log files)