I'm using the <template> tag heavily in Timelinize[0], which has a fairly sophisticated UI that I'm writing in vanilla JS -- not even jQuery. I use a few libraries (for mapping, datetime, and Bootstrap+Tabler for some nice styles), but that's it.
I wrote some boilerplate JS, yes, but I have complete control over my frontend, and I understand how it works, and I don't need to compile it.
Anyway, it works well so far! The <template> tag is a great way to lay out components and then fill them in with simple JS functions.
One nuance is documented in my code comments:
// Ohhhhhh wow, we need to use firstElementChild when cloning the content of a template tag (!!!!):
// https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template#avoiding_documentfragment_pitfalls
// I spent way too long on this.
const elem = $(tplSelector);
if (!elem) return;
return elem.content.firstElementChild.cloneNode(true);
Once I figured that out, it's been smooth sailing and very powerful.
Oh, but I haven't cleaned up my code at all yet because I was just experimenting/hustling, so it's a bit spaghetti :) If you go looking, don't mind the mess.
---
[0]: https://github.com/timelinize/timelinize - a self-hosted application for bringing together all your data on your own computer and organizing it onto a single timeline: social media accounts, photo libraries, text messages, GPS tracks, browsing history, and more. Almost ready for its debut...
You should use document.importNode() to clone templates.
Template contents are in a separate document from the main document, which is what makes them inert. importNode() adopts the nodes into the document so they have the correct prototype immediately (which includes custom elements). Otherwise the adopt steps are run when the elements are first attached to the document, which adds another tree walk to the insert/append operation that costs some time.
So:
document.importNode(elem.content, true);
Then you'll have a DocumentFragment you can pull nodes out of. Or just append the whole fragment.
I considered vanilla template initially for building my ActivityPub frontend[1], but in the end the sugar that lit-js adds on top of web components seemed worth the file size overhead.
Yeah, Lit's tagged template literals and render() method are basically a shorthand for making a <template> element, marking the spots where expressions go, cloning the template, then filling in those sports.
I haven't seen that, actually -- looks very nice. We do render a heatmap already, but I'm learning that heatmaps are very tricky to tune to be most useful/interesting. I will try to learn from that project.
Dude, cool project! I considered building something like this a couple years ago as the only way in hell I'd ever use it is if it's open source and self-hosted (thanks so much for using AGPLv3!). I ended up stuck in analysis paralysis, so I'm super impressed :-)
Ha, thanks. It's hard, so no shame for being "stuck". I've iterated on this for over a decade at this point. Still more evolution to come... but I really want this for my family.
Templates are solid and are slightly more performant than including your html-in-js for webcomponents, but that said I really wish there was some sort of web component file format that incorporated:
- The HTML
- The JS
- The CSS
In some sort of a structured way that's easy to load and distribute as a library. I don't like that to use webcomponents the right way, you have to load the logic from an import and then inline the html aspects of it.
There is a spec issue open for HTML Modules [1] along with a few proposals. The concept needs some motivated champions to see it through. Microsoft has shown some interested in this lately.
There are userland single-file component projects, like my Heximal[2] project. Though Hexiaml specifically doesn't tackle the module format - it requires some kind of HTML Module polyfill.
I don't think the current situation is so bad though. The container format should mostly be irrelevant to consumers of a component. You need to import the component defintion whether that definition is in a .js file or .html file. Once you import it you use it the same.
There should be very few cases where the file format matters. Maybe for use when JS is turned off if HTML-based web components are considered safe to evaluate (their templates might expressive enough to potentially allow attacks).
I've been using template elements a lot recently to render products on my wife's photography blog. The product data gets loaded from a headless MedusaJS instance, plugged into a template that matches her blog's theme, and then dropped into the page. I specifically wanted to avoid reaching for something like React since the page load is already pretty heavy with the size of her images. Template's been pretty easy to use in that respect.
Shopify heavily uses <template> elements with their Storefront Web Components project, except the templates themselves are passed as a child to the web component, that dynamically queries the data necessary to render the template, before mounting the element to the DOM: https://shopify.dev/docs/api/storefront-web-components
<script> parses its contents as text, whereas <template> parses as DOM. This means you don't have to escape `<`, just `</script>`.
Myself and some browser engineers been working on proposals to allow for inline modules, including JSON, that are importable into other modules via regular import statements.
This is why I recommend the "-json" type - so it doesn't collide with a future native "json" type.
My understanding is that in implementations any unknown type creates a "data block", which is just unprocessed text.'
I wouldn't use application/json just in case browsers start supporting that and it has different semantics than whatever custom thing you might do, causing a webcompat issue when the native feature rolls out.
Although with JSON, it's pretty unlikely that there would be any differing semantics. JSON modules in JS are just JSON blocks with no special additions and no named exports. That's what inline versions would be as well.
But wouldn't a subtype like `mytype` in my example (`application/mytype+json`) still be be a valid mime-type and still avoid your concerns? I've used these before.
I can imagine if it's deeply nested data, your markup might get complex, but the structure of the HTML doesn't have to mirror the JSON. There's other examples here which might illustrate more how this can look: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...
Thank you. I was looking at a generic way to pass configuration data to custom elements where I cannot know the shape in advance, since another dev can have different requirements. I support data attributes for simple types but i was wondering if allowing arbitraty json objects via other means could be feasible.
Since JSON is a subset of JavaScript, you can just use a script element, and including the json mime type certainly doesn't hurt:
<script type="application/json" id="myJSONData">
{
"name": "John Doe",
"age": 30
}
</script>
<script>
const data = JSON.parse(document.getElementById('myJSONData').textContent);
console.log(data.name + " is " + data.age); // John Doe is 30
</script>
Note: The JSON data must exist literally within the script element. If you try using a src attribute to request it from elsewhere, the response will not be available via textContent or any other mechanism [0], and at that point you just need to fetch/ajax/xhr it.
I wonder if the browser would attempt to validate the contents of a script tag with type json, versus treating it as a blob that would only be validated when parsed/used. And any performance overhead at load time for doing so. Not at a machine at the moment so I can't verify.
How so? I don't remember ever having seen issues with this. If anything CSP steers you towards this (instead of inline scripts directly assigning to JS variables)
CSP blocks execution/inclusion, but since json does not execute and any json mimetype will not do execution there is no problem.
Any CSP-allowed other script can read that application/json script tag and decode it, but it is no different than reading any other data it has access to like any other html element or attribute.
For my portfolio (miler.codeberg.page) and the refactor&redesign I'm doing of it (that I hope to finish soon!), I feed some <template>s with data coming from a json that acts like kind of a database. I feel the process of querying nodes and setting their data is still a bit rudimentary in plain JS and it should be a pain in the ass for huge datasets or lots of nodes, but at least with modern JS it is not necessary to fetch the json with a XMLHTTPRequest or something - it can work importing it like a regular JS module, so at least there's that.
Reading the first example I'm not really seeing what gains I get from this. It's just as much work to create a table row using the template as doing it from scratch.
Of course, the goal of the example is to teach using an example that makes sense logically. Templates can be arbitrarily large and obviously as the template gets bigger, the difference between it and manual `document.createElement` calls gets much larger.
Think of a large template with many attributes like classes and structure.
When you want to render this multiple times you just have to update a few things for the unique instance instead of recreating the structure and classes over and over.
I agree, this seems more complicated than just defining your template in JS. You have to use JS to interact with them anyway and it's basically a mapping between some well-known strings and a factory to create a DOM node except less flexible. The advantage seems to be only if you don't interact with your templates in JS and use them inline in your HTML.
What exactly is the point of <template> tag? Just to create document fragments? I mean that’s all you can really do with it in HTML. If you are using JS you can attach shadow dom to any element.
I guess maybe slots for web components is the other point?
I’ve used it to have the server generate fragments not processed in the normal page load which are then used as templates by JavaScript which populates them. That supported fast initial page loads compared to an SPA while being just as dynamic after being loaded.
This worked pretty well but definitely had some rough edges which I really wish the standards process could’ve focused on. Things like JSX are so much slower / memory intensive but people use them for convenience and it’d be nice to improve the ease-of-use story.
Largely, I think. Though I've not got around to actually using them yet myself, so take my understanding with a pinch of salt.
Compared to doing that with non-displayed HTML tags outside a template there are performance benefits (the parser deals with it differently knowing it isn't intended to be an active bit of DOM yet) and convenience benefits (IIRC the contents are not going to be picked up by querySelector/querySelectorAll).
Compared to building document fragments in JS they can be a lot more convenient to create and edit, and keep your design and code less tightly coupled (so a designer on your team who doesn't want to touch JS (or who you don't want messing with your JS!) can easily play with them to a certain extent). Other methods to achieve the same separation may involve an extra build step in deploying your work.
Good question. Yes there was all that web components stuff that didn't go the HTML5 way, and the template element is definitely a child of that. Since elements that get into the spec can't be removed easily, they do get repurposed instead over time.
There is untapped potential for building modern web sites by leveraging standard web technologies, such as the template element, and sidestepping a whole bunch of bloated abstractions that plague most modern web frameworks. Essentially, we need the simplicity of old school web development married with modern web features.
There are a few modern libraries and frameworks that trend in this direction: Nue, Datastar, Lit. The DX and UX with these is a breath of fresh air.
I've experimented with this approach myself with Miu[1]. It doesn't quite integrate with Web Components nicely yet, but it's almost there. I have some bones to pick with the WC standard, but that's a separate matter.
My wish is that modern web development can eventually not require any 3rd party libraries or frameworks, just like in the early days. Web developers got used to these because of limitations of native web APIs, but the web has grown leaps and bounds since then. Before the modern workflow became standard, it was magical to open a text editor, and have a functioning web page within a few minutes. No dependencies, no build step, no nonsense. Let's bring that back, please.
I would be very interested in an API similar to the plates[1] library, but with support for event bindings (instead of being just a one-off rendering of plain HTML). My first thought is that it might not be very maintainable, and at best might only be useful for small templates or templates that are heavy on microdata[2].
But there's just something about this type of binding that makes me want to give it a serious chance, at least once, before completely giving up on it.
I wrote some boilerplate JS, yes, but I have complete control over my frontend, and I understand how it works, and I don't need to compile it.
Anyway, it works well so far! The <template> tag is a great way to lay out components and then fill them in with simple JS functions.
One nuance is documented in my code comments:
Once I figured that out, it's been smooth sailing and very powerful.Oh, but I haven't cleaned up my code at all yet because I was just experimenting/hustling, so it's a bit spaghetti :) If you go looking, don't mind the mess.
---
[0]: https://github.com/timelinize/timelinize - a self-hosted application for bringing together all your data on your own computer and organizing it onto a single timeline: social media accounts, photo libraries, text messages, GPS tracks, browsing history, and more. Almost ready for its debut...
Template contents are in a separate document from the main document, which is what makes them inert. importNode() adopts the nodes into the document so they have the correct prototype immediately (which includes custom elements). Otherwise the adopt steps are run when the elements are first attached to the document, which adds another tree walk to the insert/append operation that costs some time.
So:
Then you'll have a DocumentFragment you can pull nodes out of. Or just append the whole fragment.Update: I'm using importNode() now -- works great too.
[1] https://github.com/mariusor/oni
Yeah, Lit's tagged template literals and render() method are basically a shorthand for making a <template> element, marking the spots where expressions go, cloning the template, then filling in those sports.
Regarding the gpx support... Don't want to lead you astray, but, have you seen https://www.sethoscope.net/heatmap/?
I haven't seen that, actually -- looks very nice. We do render a heatmap already, but I'm learning that heatmaps are very tricky to tune to be most useful/interesting. I will try to learn from that project.
Still highly unstable between versions, but for being free, it looks really good and is very flexible. I've been pleased with it for this project.
- The HTML
- The JS
- The CSS
In some sort of a structured way that's easy to load and distribute as a library. I don't like that to use webcomponents the right way, you have to load the logic from an import and then inline the html aspects of it.
There are userland single-file component projects, like my Heximal[2] project. Though Hexiaml specifically doesn't tackle the module format - it requires some kind of HTML Module polyfill.
I don't think the current situation is so bad though. The container format should mostly be irrelevant to consumers of a component. You need to import the component defintion whether that definition is in a .js file or .html file. Once you import it you use it the same.
There should be very few cases where the file format matters. Maybe for use when JS is turned off if HTML-based web components are considered safe to evaluate (their templates might expressive enough to potentially allow attacks).
[1]: https://github.com/WICG/webcomponents/issues/645
[2]: https://heximal.dev/ (Sorry the site is messed up on mobile. I need to fix it!)
You can also add script tags wherever you want. They will execute when added to the DOM, but they won't be scoped.
You might be thinking of shadow DOM: if you clone your template into a ShadowRoot, then any included styles will be scoped to that ShadowRoot.
<script> parses its contents as text, whereas <template> parses as DOM. This means you don't have to escape `<`, just `</script>`.
Myself and some browser engineers been working on proposals to allow for inline modules, including JSON, that are importable into other modules via regular import statements.
This is why I recommend the "-json" type - so it doesn't collide with a future native "json" type.
I wouldn't use application/json just in case browsers start supporting that and it has different semantics than whatever custom thing you might do, causing a webcompat issue when the native feature rolls out.
Although with JSON, it's pretty unlikely that there would be any differing semantics. JSON modules in JS are just JSON blocks with no special additions and no named exports. That's what inline versions would be as well.
[0] https://stackoverflow.com/questions/17124713/read-response-o...
Safe JSON in script tags: How not to break a site - https://sirre.al/2025/08/06/safe-json-in-script-tags-how-not...
As with all untrusted content, depending on where the JSON string comes from, sanitizing the output is worth considering.
Any CSP-allowed other script can read that application/json script tag and decode it, but it is no different than reading any other data it has access to like any other html element or attribute.
Maybe a more understandable example for you would be it's use in conjunction with <slot>, shown here: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...
I guess maybe slots for web components is the other point?
This worked pretty well but definitely had some rough edges which I really wish the standards process could’ve focused on. Things like JSX are so much slower / memory intensive but people use them for convenience and it’d be nice to improve the ease-of-use story.
Largely, I think. Though I've not got around to actually using them yet myself, so take my understanding with a pinch of salt.
Compared to doing that with non-displayed HTML tags outside a template there are performance benefits (the parser deals with it differently knowing it isn't intended to be an active bit of DOM yet) and convenience benefits (IIRC the contents are not going to be picked up by querySelector/querySelectorAll).
Compared to building document fragments in JS they can be a lot more convenient to create and edit, and keep your design and code less tightly coupled (so a designer on your team who doesn't want to touch JS (or who you don't want messing with your JS!) can easily play with them to a certain extent). Other methods to achieve the same separation may involve an extra build step in deploying your work.
There are a few modern libraries and frameworks that trend in this direction: Nue, Datastar, Lit. The DX and UX with these is a breath of fresh air.
I've experimented with this approach myself with Miu[1]. It doesn't quite integrate with Web Components nicely yet, but it's almost there. I have some bones to pick with the WC standard, but that's a separate matter.
My wish is that modern web development can eventually not require any 3rd party libraries or frameworks, just like in the early days. Web developers got used to these because of limitations of native web APIs, but the web has grown leaps and bounds since then. Before the modern workflow became standard, it was magical to open a text editor, and have a functioning web page within a few minutes. No dependencies, no build step, no nonsense. Let's bring that back, please.
[1]: https://github.com/hackfixme/miu
The time is right for a DOM templating API: https://justinfagnani.com/2025/06/26/the-time-is-right-for-a...
What should a native DOM templating API look like?: https://justinfagnani.com/2025/06/30/what-should-a-dom-templ...
But there's just something about this type of binding that makes me want to give it a serious chance, at least once, before completely giving up on it.
[1]: https://github.com/flatiron/plates
[2]: https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Mic...