https://css-tricks.com/named-element-ids-can-be-referenced-as-javascript-globals/ Skip to main content CSS-Tricks * Articles * Videos * Almanac * Newsletter * Guides * DigitalOcean * DO Community Search global scope JavaScript Named Element IDs Can Be Referenced as JavaScript Globals Avatar of Matteo MazzaroloAvatar of Matteo Mazzarolo Matteo Mazzarolo on Sep 27, 2022 DigitalOcean joining forces with CSS-Tricks! Special welcome offer: get $200 of free credit. Did you know that DOM elements with IDs are accessible in JavaScript as global variables? It's one of those things that's been around, like, forever but I'm really digging into it for the first time. If this is the first time you're hearing about it, brace yourself! We can see it in action simply by adding an ID to an element in HTML:
Normally, we'd define a new variable using querySelector("#cool") or getElementById("cool") to select that element: var el = querySelector("#cool"); But we actually already have access to #cool without that rigamorale: CodePen Embed Fallback So, any id -- or name attribute, for that matter -- in the HTML can be accessed in JavaScript using window[ELEMENT_ID]. Again, this isn't exactly "new" but it's really uncommon to see. As you may guess, accessing the global scope with named references isn't the greatest idea. Some folks have come to call this the "global scope polluter." We'll get into why that is, but first... Some context This approach is outlined in the HTML specification, where it's described as "named access on the Window object." Internet Explorer was the first to implement the feature. All other browsers added it as well. Gecko was the only browser at the time to not support it directly in standards mode, opting instead to make it an experimental feature. There was hesitation to implement it at all, but it moved ahead in the name of browser compatibility (Gecko even tried to convince WebKit to move it out of standards mode) and eventually made it to standards mode in Firefox 14. One thing that might not be well known is that browsers had to put in place a few precautionary measures -- with varying degrees of success -- to ensure generated globals don't break the webpage. One such measure is... Variable shadowing Probably the most interesting part of this feature is that named element references don't shadow existing global variables. So, if a DOM element has an id that is already defined as a global, it won't override the existing one. For example:
I won't override window.foo
And the opposite is true as well:
I will be overridden :(
This behavior is essential because it nullifies dangerous overrides such as
, which would otherwise create a conflict by invalidating the alert API. This safeguarding technique may very well be the why you -- if you're like me -- are learning about this for the first time. The case against named globals Earlier, I said that using global named elements as references might not be the greatest idea. There are lots of reasons for that, which TJ VanToll has covered nicely over at his blog and I will summarize here: * If the DOM changes, then so does the reference. That makes for some really "brittle" (the spec's term for it) code where the separation of concerns between HTML and JavaScript might be too much. * Accidental references are far too easy. A simple typo may very well wind up referencing a named global and give you unexpected results. * It is implemented differently in browsers. For example, we should be able to access an anchor with an id -- e.g. -- but some browsers (namely Safari and Firefox) return a ReferenceError in the console. * It might not return what you think. According to the spec, when there are multiple instances of the same named element in the DOM -- say, two instances of
-- the browser should return an HTMLCollection with an array of the instances. Firefox, however, only returns the first instance. Then again, the spec says we ought to use one instance of an id in an element's tree anyway. But doing so won't stop a page from working or anything like that. * Maybe there's a performance cost? I mean, the browser's gotta make that list of references and maintain it. A couple of folks ran tests in this StackOverflow thread, where named globals were actually more performant in one test and less performant in a more recent test. Additional considerations Let's say we chuck the criticisms against using named globals and use them anyway. It's all good. But there are some things you might want to consider as you do. Polyfills As edge-case-y as it may sound, these types of global checks are a typical setup requirement for polyfills. Check out the following example where we set a cookie using the new CookieStore API, polyfilling it on browsers that don't support it yet: This code works perfectly fine in Chrome, but throws the following error in Safari.: TypeError: cookieStore.set is not a function Safari lacks support for the CookieStore API as of this writing. As a result, the polyfill is not applied because the img element ID creates a global variable that clashes with the cookieStore global. JavaScript API updates We can flip the situation and find yet another issue where updates to the browser's JavaScript engine can break a named element's global references. For example: That script grabs a reference to the input element and invokes focus () on it. It works correctly. Still, we don't know how long it will continue to work. You see, the global variable we're using to reference the input element will stop working as soon as browsers start supporting the BarcodeDetector API. At that point, the window.BarcodeDetector global will no longer be a reference to the input element and .focus() will throw a "window.BarcodeDetector.focus is not a function" error. Conclusion Let's sum up how we got here: * All major browsers automatically create global references to each DOM element with an id (or, in some cases, a name attribute). * Accessing these elements through their global references is unreliable and potentially dangerous. Use querySelector or getElementById instead. * Since global references are generated automatically, they may have some side effects on your code. That's a good reason to avoid using the id attribute unless you really need it. At the end of the day, it's probably a good idea to avoid using named globals in JavaScript. I quoted the spec earlier about how it leads to "brittle" code, but here's the full text to drive the point home: As a general rule, relying on this will lead to brittle code. Which IDs end up mapping to this API can vary over time, as new features are added to the web platform, for example. Instead of this, use document.getElementById() or document.querySelector(). I think the fact that the HTML spec itself recommends to staying away from this feature speaks for itself. DigitalOcean joining forces with CSS-Tricks! Special welcome offer: get $200 of free credit. Comments 1. Reed Permalink to comment# September 27, 2022 The feature doesn't seem to work for elements inside shadow DOM. Reply 2. Ben Nadel Permalink to comment# September 27, 2022 Super interesting! I actually ran into this a while back, but had no idea what was happening. We had a Script tag that was like: But, whenever I tried to access window.config, I was getting the Script-tag Reference, and NOT the inline-object. I had no idea what was going on, and just ended up removing the id attribute (since it wasn't doing anything). Good to know there was at least some logic as to why this was happening. Reply + Matteo Permalink to comment# September 27, 2022 Hey Ben! :D Weird! window.config should have worked in your example. Wondering what could have been. 3. Chris Morgan Permalink to comment# September 27, 2022 It is implemented differently in browsers. For example, we should be able to access an anchor with an id -- e.g. -- but some browsers (namely Safari and Firefox) return a ReferenceError in the console. Huh? There's nothing special about particular tag names, and any browser that that didn't work in would be violating the spec. Certainly it works just fine in Firefox. I have no idea where this came from. As a matter of fact, I did find one clear spec violation in this very feature last week (which I haven't bothered to file at this time): Chromium doesn't implement it if the document is loaded with XML syntax. So this first URL throws a ReferenceError in Chromium (unlike Firefox, or either in the second URL): data:application/xhtml+xml, data:text/html, All up, it's one of my favourite aggressive minification/golfing tricks (and I use it regularly in one-off scripts for my own use), but it's not the most robust. Reply 4. Chris Morgan Permalink to comment# September 27, 2022 Want to hear something funny? To add insult to the injury, named elements are accessible as global variables only if the names contain nothing but letter. Browsers won't create a global reference for an element with a ID that contains special characters and numbers, like hello-world and item1. This is completely false. window["hello-world"] and item1 will both work just fine. (Proof: go to data:text/html,

and try it.) This and a few other things in the article suggest that the author hasn't understood how this whole thing is actually implemented (which is admittedly not awfully clear from the spec if you don't understand JavaScript's prototypal inheritance model or the nature of the window global object). Without getting into the details: this isn't the browser setting properties when you provide an element with such-and-such an ID, but rather the fallback, so that when you try accessing a property that doesn't exist on the global object, it will try finding a suitable object to give you, instead of giving you undefined straight away. Reply + Matteo Permalink to comment# September 27, 2022 Yeah, that bonus point shouldn't have slipped in -- my bad. Thanks for reporting! I'll also double-check what you pointed out in the other comment. 5. Manuel Strehl Permalink to comment# September 27, 2022 Additional "fun fact": The same mechanism is available on the HTMLFormElement interface, i.e. on all

nodes. If you are so unlucky to give any button or input element the ID submit and then try to call form.submit() you're in for a really bad surprise:
Reply 6. Reed Permalink to comment# September 27, 2022 I wonder what was actually happening in your case, because according to the blog post the value of window.config should be the inline-object. Reply Leave a Reply Cancel reply Your email address will not be published. Required fields are marked * [ ] [ ] [ ] [ ] [ ] [ ] [ ] Comment * [ ] Name * [ ] Email * [ ] Website [ ] [ ] Save my name, email, and website in this browser for the next time I comment. [ ]Get the CSS-Tricks newsletter [Post Comment] Copy and paste this code: micuno * [4.1 ] Leave this field empty [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] D[ ] CSS-Tricks is powered by DigitalOcean. Keep up to date on web dev with our hand-crafted newsletter DigitalOcean * DigitalOcean * DigitalOcean Community * About DigitalOcean * Legal * Free Credit Offer CSS-Tricks * Email * Guest Writing * Book * Advertising Follow * Twitter * Instagram * YouTube * CodePen * iTunes * RSS Back to Top [Jetpack-Search-on-CSS-Tricks-Instant-Search]