Getting error on lap counter script JS

I am getting the following error Assets/LapCounter.js(4,13): BCE0034: Expressions in statements must only be executed for their side-effects. and i dont know why here is my script (i am a newbie)

 #pragma strict
 var Lap: int;
 function Start () {
     Lap == 0;
 }
 
 function Update () {
     if (Lap ==3);    
 }
 
 function OnCollisionEnter(collision: Collision) 
 {
  Lap++;
 }