What is the syntax for creating a function in javascript which can modify the value of a given parameter ?

For example, the unity function Vector3.Smoothdamp modifies "currentVelocity"

The function signature is (from Unity scripting manual):

static function SmoothDamp (current : Vector3, target : Vector3, ref currentVelocity : Vector3, smoothTime : float, maxSpeed : float = Mathf.Infinity, deltaTime : float = Time.deltaTime) : Vector3

The function uses the "ref" keyword to (apparently) indicate what looks like C-style indirection (i.e., *currentVelocity). The "ref" keyword is also a feature of C# (as well as "out"), which produces the same effect in C#.

However, when I try to write a javascript function using "ref" in the same manner as shown in the unity docs, I get an error: BCE0043: "unexpected token: currentVelocity"