Put A Function Inside A Function Properly?

I need to create a code that takes in a function f and a variable x and gives me f(f(x)). For example, if f(x)=2*x and x=3, then f(f(x))=12.

I've tried to look at this from two angles: creating an extra variable to define the function within the function and reusing f on itself. But I can't use either properly.

How can I get my main function to accept a function as its first parameter [function(2*x, 3)]? Is that even possible? I could specify the function f within the twoTimes function, but the point is for f to be a changeable parameter.

I had come up with, but it definitely doesn't work. How should I approach it?

var twoTimes = function(f,x) {
f = function(x) {};
var r = f(x);
return f(r);
};
twoTimes(2*x, 3)