Fourier Series Fitting using Fit function

Dear All;

There is a command to fit different fucntion to data , this command called "Fit" which is also used to fit data to fourier series but when i check types of functions that command fit data into , i could find " fourier3 ... fourier8" and even when i check "Fit" function code i could not find it , any one can helo why is not shown in code and how can i get it?