How can i get window property from element

Question

<body>
<h1 id="a">Site 3</h1>

<script>

var arr = []

for(var i in window)
{
    arr.push(i) 
}
console.log("a" in window)// true
console.log(arr)// dont have property a why?!
</script>

</body>

But when I run console.log(arr) I don't have a in the array, why?

look at my code and if you know answer me why arr dont have element "a" — Mankament Gra, Oct 30 '16 at 17:28
This would be browser specific, the specs just say that *"the global object may have additional host defined properties..."*, and named elements being part of the global object is up to the vendor, and was introduced by Microsoft many years ago, is not standardized, and can be implemented differently — adeneo, Oct 30 '16 at 17:31
@adeneo: No, it's in the spec now: https://www.w3.org/TR/html5/browsers.html#named-access-on-the-window-object — T.J. Crowder, Oct 30 '16 at 17:31
@ScottMarcus: Because when you put an `id` on an element, the browser creates an automatic global for it, and because until ES2015 all globals had to be properties of the global object (which `window` references on browsers), you end up with `a` on `window` being a reference to the `div` with that ID. — T.J. Crowder, Oct 30 '16 at 17:32
I didn't see the id=a. My bad. Turns out to just be an issue of an own property or not. — Scott Marcus, Oct 30 '16 at 17:33
@T.J.Crowder - oh, that's right, I always forget that HTML5 standardized that shit, it even says so in the old [question](http://stackoverflow.com/questions/3434278/do-dom-tree-elements-with-ids-become-global-variables) about this. — adeneo, Oct 30 '16 at 17:35

T.J. Crowder · Accepted Answer · 2016-10-30T17:33:31.323

2

The things accessible on window are not necessarily "own" properties of window and (this is the significant bit) are not necessary enumerable. for-in only visits enumerable properties (both "own" and inherited).

On Chrome, for instance, the automatic a global is a property of the prototype of the prototype of window, and it's marked as non-enumerable:

var o = window;
var where = "window"
while (o && !Object.getOwnPropertyDescriptor(o, "a")) {
  where = "prototype of " + where;
  o = Object.getPrototypeOf(o);
}

console.log(where);
console.log(Object.getOwnPropertyDescriptor(o, "a"));

<div id="a"></div>

Looks like it's in the same place on Firefox.

The exact semantics of how named access is achieved isn't dictated, so long as in works and window.a works.

edited Oct 30 '16 at 17:33

answered Oct 30 '16 at 17:28

T.J. Crowder

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ok, so why "a" is in prototype of prototype window rather than window? – Mankament Gra Oct 30 '16 at 20:38
@MankamentGra: According to the above it's on the *prototype* of the prototype of `window`. I don't know, at that point we're into implementation details, but `window` is **very** complicated, taking things from lots of different places, incorporating the [`WindowProxy` object](https://www.w3.org/TR/html5/browsers.html#the-windowproxy-object), etc. I'm not surprised to find three layers to it. – T.J. Crowder Oct 31 '16 at 07:42

How can i get window property from element

1 Answers1