How to keep object from going off screen

 var moveDirection = Vector3(Input.GetAxis("Horizontal"),Input.GetAxis("Vertical"), 0);
     rigidbody.MovePosition(transform.position + moveDirection*moveSpeed*Time.deltaTime);

here is my movement script and i want to make it so the character can ply move if he is within the bounds of the screen.

     #pragma strict
     
     var moveSpeed : float = 1.0;
     var Health : int = 20;
     private var Ammo : GameObject;
     
     var bullet : GameObject;
     var bomb : GameObject;
     var misile : GameObject;
     var sheild : GameObject;
     var PowerParticles : GameObject[];
     
     var bulletSpeed : float = 1.0;
     var shootDelay : float = 0.2;
     
     public var Fire : AudioClip;
     public var Hurt : AudioClip;
     public var Power : AudioClip;
     var HealthText : GameObject;
     var number : int = 1;
     
     private var canShoot : boolean = true;
     private var i : int = 0;
     private var e : int = 0;
     
     function Start(){
     Ammo = bullet;
     }    
     
     function FixedUpdate (){
         HealthText.GetComponent(TextMesh).text = (Health.ToString());
         if (Health <= 0){
             Playerdie();
         }                
         var moveDirection = Vector3(Input.GetAxis("Horizontal"),Input.GetAxis("Vertical"), 0);
         rigidbody.MovePosition(transform.position + moveDirection*moveSpeed*Time.deltaTime);
         if ((Input.GetAxis("FireHorizontal") != 0.0f || Input.GetAxis("FireVertical") != 0.0f) && canShoot){
             audio.PlayClipAtPoint(Fire, Vector3 (0, 0, 0));
             var shootDirection = Vector3(Input.GetAxis("FireHorizontal"),Input.GetAxis("FireVertical"), 0).normalized;
             var AmmoInstance = Instantiate(Ammo,transform.position,Quaternion.LookRotation(shootDirection));
             Destroy (AmmoInstance, 10);
             AmmoInstance.rigidbody.AddForce(shootDirection*bulletSpeed,ForceMode.VelocityChange);
             canShoot = false;
             Invoke("ShootDelay", shootDelay);
         }
     }
     
     function OnCollisionEnter(collision : Collision) {
         rigidbody.velocity = Vector3.zero;
         if (collision.gameObject.tag == "Enemy"){
             Damage();
         }
         if (collision.gameObject.tag == "Health"){
         var Healthp = Instantiate(PowerParticles[0], transform.position, transform.rotation);
         Destroy (Healthp, 1);
         audio.PlayClipAtPoint(Power, Vector3 (0, 0, 0));
             Heal();
             Destroy(collision.gameObject,0);
         }
         if (collision.gameObject.tag == "Machinegun"){
         var MachineP = Instantiate(PowerParticles[1], transform.position, transform.rotation);
         Destroy (MachineP, 1);
         audio.PlayClipAtPoint(Power, Vector3 (0, 0, 0));
         Destroy(collision.gameObject,0);
         shootDelay = 0;
         yield WaitForSeconds(1);
         shootDelay = 0.2;
         }
         if (collision.gameObject.tag == "Bomb"){
         var Bombp = Instantiate(PowerParticles[2], transform.position, transform.rotation);
         Destroy (Bombp, 1);
         audio.PlayClipAtPoint(Power, Vector3 (0, 0, 0));
         Destroy(collision.gameObject,0);
         Ammo = bomb;
         yield WaitForSeconds(1);
         Ammo = bullet;
         }
         if (collision.gameObject.tag == "Seekingmissile"){
         var Missilep = Instantiate(PowerParticles[3], transform.position, transform.rotation);
         Destroy (Missilep, 1);
         audio.PlayClipAtPoint(Power, Vector3 (0, 0, 0));
         Destroy(collision.gameObject,0);
         Ammo = misile;
         yield WaitForSeconds(1);
         Ammo = bullet;
         }
         if (collision.gameObject.tag == "Shield"){
         audio.PlayClipAtPoint(Power, Vector3 (0, 0, 0));
         Destroy(collision.gameObject,0);
         Shield();
         }
     }
     
     function Shield(){
         var    currentshield = Instantiate(sheild, transform.position, transform.rotation);
         currentshield.transform.parent = transform;
         Destroy (currentshield, 1.5);
     }
         
     function OnCollisionExit(collision : Collision){
         rigidbody.velocity = Vector3.zero;
     }
     
     function ShootDelay(){
         canShoot = true;
     }
     
     function Damage(){
         if (Health > 0){
             audio.PlayClipAtPoint(Hurt, Vector3 (0, 0, 0));
             Health--;
         }
     }
     
     function Heal(){
         if (Health < 40){
             Health++;
         }
     }
     
     function Playerdie(){
         yield WaitForSeconds(1);
         Application.LoadLevel("menu");
     }