how can convert this in js?

/C#/Vector3 position { get{ return m_Position; } }
/JS/var position : Vector3 = ?;

/*C#*/public abstract void Solve(Transform[] bones, Vector3 target);
/*JS*/public abstract function Solve(bones : Transform[], target : Vector3); // ERROR

/*C#*/public abstract int samples {get;}
/*JS*/public abstract var samples : int = ?;

it’s from Locomotion System

For #1:

private var m_Position:Vector3;

function get position:Vector3 { 
     return m_Position;
}
function set position(value:Vector3) {
    m_Position = value;
}

For #2 and #3: Unity Javascript doesn’t support abstract functions, as far as I know. For the function, leave out abstract & just use an empty implementation. For the second, it’s another property so follow the pattern for #1.