Spawning won't work

I am writing a script to spawn a mass of cubes when I press X. When I try to run it unity say unexpected token.

 // JavaScript
 
 
 
 
 function Start ()
 if(Input.GetButtonDown("X"))        
 
  {
     for (var y = .5; y < 10; y++) {
         for (var x = 0; x < 10; x++) {
              for (var z = 0; z < 10; z++) {
             
             var cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
            
              cube.AddComponent(Rigidbody);
             cube.rigidbody.mass = 10;
             cube.transform.position = Vector3 (x, y, z);
      print ("made");
  
                        }
     }
     
 }
     
 }