variables in OnCollisionEnter different to actual variables

The values of my class variables in OnCollisionEnter2D are different to what the values actually are, they always seem to be false, 0 or null (I think the default values).

 var testNo : int;
 
 function Start () {
     testNo = 5;
     Debug.Log(testNo);
     
 }

 function Update () {

 if(Input.GetKeyDown("u")){
     Debug.Log(testNo);
 }

 function OnCollisionEnter2D(collider : Collision2D){
     Debug.Log(testNo);
 }

and the output is 5 at the start and then when OnCollisionEnter gets called 0 is printed. If I press "u" after OnCollisionEnter is called 5 is printed.