Closure Compiler Externs - WARNING - Property never defined on - google-closure-compiler

Question

I am preparing externs for PIXI.js library. I am getting the following warning:
js/Test.js:188: WARNING - Property position never defined on PIXI.Sprite
button.position.y = y;
Here are the relevant extern definitions:
//UPDATE
/**
* #constructor
* #extends {PIXI.Container}
* #param {PIXI.Texture} texture
*/
PIXI.Sprite = function(texture){};
/**
* #constructor
* #extends {PIXI.DisplayObject}
*/
PIXI.Container = function(){};
/**
* #constructor
* #extends {PIXI.EventEmitter}
*/
PIXI.DisplayObject = function(){};
/**
* #type {PIXI.Point}
*/
PIXI.DisplayObject.position;
Still getting the same warning.
What am I doing wrong?
When I am replacing PIXI.DisplayObject.position; with PIXI.DisplayObject.prototype.position; that seems to clear the warning.
Does it mean that I should always define SomeObject.prototype.prop rather then SomeObject.prop ?

Answer 1

This is highlighting the difference between static and prototype properties.
Given:
/**
* #constructor
* #param {number=} opt_num
*/
function foo(opt_num) {
if (opt_num !== undefined) {
this.bar = opt_num;
}
}
foo.prototype.bar = 17;
foo.bar = 42;
We have both a static property and a prototype property of the same name. However they are referenced differently:
console.log((new foo()).bar); // 17
console.log((new foo(0)).bar); // 0
console.log(foo.bar); // 42
So in an extern, when you are defining properties on a type - you typically want to define them on the prototype object:
/** #param {foo} obj_foo */
function log(obj_foo) {
// This is an instance of "foo".
// The "bar" property references prototype or instance
// properties - not static properties.
console.log(obj_foo.bar);
// Static properties can only be referenced by the full namespace
console.log(foo.bar);
}