I need help with Animator and JavaScript

this is my script …

Code (JavaScript):

1. var anim : Animator;
2. var alter: GameObject;
3.  
4. function Start () {
5.      anim = alter.GetComponent("Animator");
6. }
7.  
8. function Update () {
9.      if(Input.GetKeyDown("z"))
10.     {
11.         anim.SetTrigger("select");
12.     }
13.    
14.     if(Input.GetKeyDown("b"))
15.     {
16.         anim.SetTrigger("norm");
17.     }
18. }

but I do not understand the console

Assets/select.js(7,35): BCW0028: WARNING: Implicit downcast from 'UnityEngine.Component' to 'UnityEngine.Animator'.