Unity3D JavaScript/.NET code throwing compilation error with a List-thingy [closed]

Seems like the list you are adding to is still null. In c# you would need to do something along var intList=new List<System.Int32> but I am not totally sure with JS.

I am more used to the array as a list element in JS, maybe take a look at this article

var intlist=new Array() //1) regular array. Pass an optional integer argument to control array's size.
intlist[0]=1;

Edit regarding your edit:

Just to clarify: There is a huge difference between a list beeing null or empty. When you are creating your var, you basically just tell the code that you want to use a list somewhere. This object ist still null, which means there is no memory allocated for it. You need to create the object (again in C#) usally by using the constructor. The list constructor will yield you an empty (but no longer null) list. To this list you may add objects.

This is your error, accoidng to this article about collections in unity your syntax is not wrong, but it is incomplete. See this snipped from the article:

var myList : List.<Type> = new List.<Type>();  // declaration
var someNumbers = new List.<int>();            // a real-world example of declaring a List of 'ints'
var enemies = new List.<GameObject>();         // a real-world example of declaring a List of 'GameObjects'
myList.Add(theItem);                           // add an item to the end of the List
myList[i] = newItem;                           // change the value in the List at position i
var thisItem = List[i];                        // retrieve the item at position i
myList.RemoveAt(i);                            // remove the item from position i

As you notice you need to initalize the list first via constructor, by calling it with the new keyword.

I got this fully functioning in Unity on an empty game object, you need to make sure you make the list with the new statement and also ensure you have the imports correctly defined.

#pragma strict
import System.Collections.Generic;
import System.Linq;

function Start () {
    var intlist : List.< System.Int32 > = new List.< System.Int32 >() ;
    intlist.Add(42);
    Debug.Log(intlist[0] ); //Outputs 42
}