How can I enable an object in js?

It is possible try

 var yourGameObject = GameObject.Find('your gameobject');
 
 yourGameObject.SetActive(true);

it would be easier to solve if you post an example code, personally I prefer C# but this can be achieved in both unityScript and C#

 var target : Transform;
 var distance = 3.0;
 var xSpeed = 200.0;
 var ySpeed = 200.0;
 var yMinLimit = -20.0;
 var yMaxLimit = 80.0;
 var mLimit = 0.0;
 public var ship;
 public var shipb;
 public var ship2 : GameObject;
 
 private var rightclicked = null;
 private var x = 0.0;
 private var y = 0.0;
 var minDistance = 1.0;
 var maxDistance = 20000.0;
 
 public var s1 : boolean = false;
 public var s2 : boolean = false;
 public var s3 : boolean = false;
 public var s4 : boolean = false;
 
 @script AddComponentMenu("Camera-Control/Mouse Orbit")
 
 function Start () {
     var angles = transform.eulerAngles;
     x = angles.y;
     y = angles.x;
     //Debug.Log("Hit 0.");
     //CheckActive1();
     //Debug.Log("Hit 6.");
 
     // Make the rigid body not change rotation
        if (GetComponent.<Rigidbody>())
         GetComponent.<Rigidbody>().freezeRotation = true;
 }
 
 function Update () {
         CheckActive1();
         var rotation = Quaternion.Euler(y, x, 0);
         var position = rotation * Vector3(0.0, 0.0, -distance) + target.position;
         transform.position = position;
             distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel")* mLimit, minDistance, maxDistance);
             //distance += -300.0f*Input.GetAxis ("Mouse ScrollWheel")*5, minDistance, maxDistance);
             //")*Time.deltaTime;
         if(distance < minDistance)
             distance = minDistance;
         else
         if(distance > maxDistance)
             distance = maxDistance;
         if (Input.GetMouseButtonDown(1)) {
             rightclicked = true;
         }
         if (Input.GetMouseButtonUp(1)) {
             rightclicked = false;
         }
 }
 
 function CheckActive1 () {
     Debug.Log("Hit 0.1.");
     if (s1 == true) {
         //GameObject.FindWithTag("t1").Transform.SetActive;
             Debug.Log("Hit 1.");
             //ship = GameObject.SetActive(true).FindWithTag("t1");
             //Debug.Log("Hit 1.3.");
             ship2 = GameObject.FindGameObjectWithTag("t1").SetActive(true);
             //var ship2 = GameObject.FindWithTag("t1");
             //ship2 = enabled(true);
             target = ship2.transform;
             //ship = GameObject.FindWithTag("t1").transform;
             //Debug.Log("Hit 2.");
             //target = ship;
             Debug.Log("Hit 3.");
             mLimit = 1000;
             Debug.Log("Hit 4.");
             minDistance = 7500;
             Debug.Log("Hit 5.");
             distance = 10000;
             maxDistance = 20000;
             s1 = false;
             //}
         }
     else if (s2 == true) {
         //GameObject.FindWithTag("t2").Transform.SetActive;
             Debug.Log("Hit 1b.");
             ship = GameObject.FindWithTag("t2").enabled = true;
             Debug.Log("Hit 1.2b");
             ship = GameObject.FindWithTag("t2").transform;
             Debug.Log("Hit 2b.");
             target = ship;
             Debug.Log("Ship: " + ship);
             mLimit = 1;
             Debug.Log("Mouse Limit: " + mLimit);
             distance = 3;
             Debug.Log("Camera Distance: " + distance);
             minDistance = 2.30;
             Debug.Log("Min Distance: " + minDistance);
             maxDistance = 60;
             Debug.Log("Max Distance: " + maxDistance);
             s2 = false;
             //}
         }
     }
 
 function LateUpdate () {
      if (target && rightclicked == true) {
         x += Input.GetAxis("Mouse X") * xSpeed * 0.01;
         y -= Input.GetAxis("Mouse Y") * ySpeed * 0.01;
          
          y = ClampAngle(y, yMinLimit, yMaxLimit);
                 
         var rotation = Quaternion.Euler(y, x, 0);
         var position = rotation * Vector3(0.0, 0.0, -distance) + target.position;
         
         transform.rotation = rotation;
         transform.position = position;
         }
 }
 
 static function ClampAngle (angle : float, min : float, max : float) {
     if (angle < -360)
         angle += 360;
     if (angle > 360)
         angle -= 360;
     return Mathf.Clamp (angle, min, max);
 }

As for so far I know, You cannot check (enable) your gameObject when it is unchecked (disabled in the hierarchy).

Try a different approach like disabling a component from that gameObject to get the desired effect.

EDIT : As for Sketch_Kidd answer, that only works when you try it the other way. (when you try to disable the gameObject)