##JavaScript, 34 bytes function f(n){return n?n*f(n-1):1}
(or)
function f(n){return n?n*f(--n):1} ###Explanation
Function takes in a value, returns itself multiplied by
if n != 0: the same function on the number decreased by one
if n == 0: 1
The final f(0) returns first with 1, times 1, times 2, etc.
Terminator removed, may upset use strict.