Mathematica:How can I acclaim a variable is real in a complex function

[palzle]

Hi,I'm a beginner.
here's the example:
f[x_] := E^(I*x)
Conjugate[f[x]]*f[x]
I'd like get 1,but it give me a complex function coz it regard x as complex.
please help me:how to claim x is real?

by the way,is that necessary to define a letter as a constant.no matter yes or not,how to refer a letter constant to be real or pure imaginary

Thank you!

 

[CRGreathouse]

You could do

f[x_]:=Exp[I*Re[x]]

so it takes only the real part of x (which doesn't change it, of course, when x is real).

 

[palzle]

Thanks for your reply.
I tried your answer,and it works.

Well,I have some...eerr...
what if I'd define the domain for function f[x]?
I mean that I want to prescribe x in f[x] must be real or pure imaginary or rational number.How to do that.

I've a similar question here.How to claim a letter constant to be real or pure imaginary or rational number?

Waiting for your kindness.