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Templating in HTML

Templating in HTML

77 comments

·October 4, 2022

spankalee

<template> is great, but some people are surprised that it doesn't include any form of parameterization or expressions. The entire process of cloning a template and updating it with data is left up to the developer - it's pretty low level.

That's why my team made the lit-html library, which uses JS tagged template literals to make <template> elements for you, clone them and interpolate data where the JS expressions are, and then update the result really, really fast with new data:

      import {render, html} from 'https://unpkg.com/lit?module';

      let count = 0;
      
      function increment() {
        count++;
        renderCount();
      }   
      
      function renderCount() {
        render(html`
          <p>The count is ${count}</p>
          <button @click=${increment}>Increment</button>
        `, document.body);
      }      
      renderCount();
You can try that out here: https://lit.dev/playground/#gist=1eff9baed1251fc60dd7da8b7f9...

We're also working with Apple on a proposal called Template Instantiation which brings interpolations and updates into HTML itself (though the pandemic and things really stalled that work for the moment).

voxal

lit-html is great. I use it for a lot of projects whenever I need some simple tempted HTML. That being said, it seems like the v1 documentation for lit-html [1] has been removed in favor of a single page in the v2 documentation [2]. Is using lit-html standalone not recommended anymore? I really liked using it standalone.

[1] https://lit.dev/docs/v1/lit-html/introduction/ [2] https://lit.dev/docs/libraries/standalone-templates/

spankalee

Standalone use is great and still works fine.

The organization of the lit-html and lit-element packages didn't change: they're separate and lit-element depends on lit-html. The only difference is that we added the lit package that rolls them both up and we talk about templating in one place in the docs rather than two (lit-html and lit-element used to have separate sites). We did this because most people used lit-element and the separation was confusing to some and a little bit of rough DX for most.

wildrhythms

I use lit-html for front end projects at work and it's a godsend, truly. Being able to keep one central 'state' object and just re-render a template based on that without having to pull in a whole framework, without having to set up a whole webpack build process, is incredible!

forsakenharmony

You can also use preact without a build step via htm

https://github.com/developit/htm#installation

bugmen0t

Do you have a link to the template instantiation proposal?

twsted

In the past I've seen this one:

https://github.com/WICG/webcomponents/blob/gh-pages/proposal...

Perhaps there are more recent versions.

I liked the spirit of the proposal, but never studied it.

verisimilidude

The <template> tag really shines when used alongside <slot> and the Shadow DOM.

But I really wish there were an HTML-native way to load <template> from separate files, the same way we do with CSS and JS. Not a show-stopper, but it’d be very nice to have.

goatlover

> But I really wish there were an HTML-native way to load <template> from separate files

Seems like the JS folks blocked this from happening. It's weird to me to think we've arrived at the point where JS considerations have come to dominate for something that was built to be a document sharing platform.

spankalee

No one blocked it. If you're referring to HTML Imports, the problem with them is that they didn't integrate with JS modules. There's a new HTML modules proposal [1], and with JS import assertions and CSS and JSON modules landing [2], HTML modules are probably not far behind.

[1]: https://github.com/WICG/webcomponents/blob/gh-pages/proposal... [2]: https://web.dev/css-module-scripts/

dmitriid

goatlover: Seems like the JS folks blocked this from happening.

spankaalee: not true, they were removed because.... they were't compatible with JS

---

goatlover: JS considerations have come to dominate for something that was built to be a document sharing platform.

spankalee: here are two proposals which are... Javascript-only, don't work without javascript, and add more Javascript to the platform

---

¯\_(ツ)_/¯

dmitriid

> where JS considerations have come to dominate

Not JS. Chrome, and their decade-old crusade to make web components happen, no matter the cost or common sense

cantSpellSober

Yes, I thought that's what this article would be about. Respectfully, the MDN reference for <template> has more information than this post.

null

[deleted]

brundolf

Sorry, but the <template> tag doesn't actually do "templating in HTML"

...but it should. It's shocking to me that we've had the modern paradigm of client-side-rendering frameworks for over a decade now, without so much as an RFC for any kind of native support.

For the sake of render performance, bundle size, removing a need for transpilation, compatibility across frameworks. There are a million reasons this should be happening in the browser at this point. I'm sure it's a complicated standard to come up with (for one: HTML-based vs JS-based rendering), but why does it feel like nobody's even trying?

tannhaeuser

There's no lack of syntactical templating mechanisms when HTML is hosted within a SGML markup processing context (ie how HTML started life), and no need to add templating at the HTML markup vocabulary level either:

For server-side rendering, SGML (and also some other template "engines") provide HTML-aware, type-checked, parametric macro expansion. Actually, SGML templating works transparently on the client side and the server side.

Within the browser OTOH, there's already JavaScript, making every dynamic feature added to HTML inessential for better or worse, like it has for over 25 years now. That's just how it was decided a long time ago, and adding half-assed features to HTML like the template element (which requires JavaScript, in turn, thus doesn't add any essential capability) all the time is exactly the thing we shouldn't be doing when the damn "web stack" is already bloated as fuck.

encryptluks2

Because JS is where the privacy holes are at and keep getting added.

labrador

> Let’s start with the fact that <template> do not enabling you to do anything that’s not possible otherwise. In that way, it’s more of a convenience tool really. If you have significant HTML structures that need to be injected at runtime, it might be very cumbersome to do so manually with document.createElement and element.setAttribute.

I didn't expect the paragraph to end that way. I would write it:

Let’s start with the fact that <template> do not enabling you to do anything that’s not possible otherwise. In that way, it’s more of a convenience tool really. If you have significant HTML structures that need to be injected at runtime, you can use "display:none" to hide the HTML structure, then copy node to make and show a copy, for example a dialog box

Anyway, that's how I've been doing it. If <template> has some advantages, I'll start using it

spankalee

<template> is special, very different from display: none; and definitely has a few advantages:

- The elements in it are parsed into a different document and are inert until cloned or adopted into the main document. Images don't load, scripts don't run, styles don't apply, etc. This is very important.

- The content model validation is turned off, so a <template> can contain a <td>

- Mutation observers and events are not fired for parsed content.

- The <template> element itself can appear anywhere, including restricted content elements like <table>

- Other HTML processing libraries generally know to ignore <template>, unlike other content just hidden with CSS.

This makes parsing faster and guaranteed to have no side effects, and conveys the correct semantics to the browser and tools.

tomhallett

In addition - “template” also conveys correct semantics to the other developers working on the codebase. If I see an html at the bottom with “display: none” it’s very possible that the intended usage is to leave it there and set “display: block”, ie a modal. “display: none” doesn’t convey “clone me”, but “template” does.

sgc

You can use templates for simpler items as indicated in the linked article, but it seems like it would be best used for larger datasets. I would usually load those async to avoid blocking the page while they download. Does it provide practical advantages over loading html to a js object via fetch?

Stamp01

I think the main advantages are that you don't have to remember to add "display:none", and that it gives the structure more semantic meaning. If I look at your code and see "display:none", I don't know why you've chosen to hide it. If it's wrapped in a <template> element, I instantly know why it's not being displayed as I now have an idea of how it's going to be used.

galaxyLogic

Yes but when you see "display:none;" isn't it obvious that the purpose is to make it visible sometime later?

Also if you need to use the same HTML elsewhere, just copy the innerHTML of such an element and insert anywhere you want to.

xigoi

> Yes but when you see "display:none;" isn't it obvious that the purpose is to make it visible sometime later?

Yes, but it's not obvious that it would be made visible in a different place while keeping the original invisible.

forgotusername6

Hidden forms are an example of display:none that will not be displayed later

miohtama

If I recall correctly there are some subtle things you cannot achieve without <template>.

<img> inside <template> is not attempted to be loaded because “it presents nothing in rendering”

https://html.spec.whatwg.org/multipage/scripting.html#the-te...

runarberg

> for example a dialog box

The <dialog> element is now supported by all modern browser. I suggest you use it. It has a lot of niceties over implementing one your self, including capturing the focus, correct announcing to assistive technologies, styling the ::backdrop element, etc.

labrador

Thanks for the tip. I should read the spec again so I don't miss these.

apatheticonion

I think it's mostly performance as the browser can add performance optimisations for HTML but not unevaluated JavaScript.

The alternative is a "display: none" block, but that triggers a calculation of styles, where <template> can never be rendered so it's evaluated (likely in parallel to JS) by the browser without style calculations.

The optimisations lose weight if the <template> tag is added programatically later. For it to be maximally efficient; all of the <template> tags must be inlined in your HTML prior to your application loading. You can play around with loading it asynchronously to ensure nothing blocks.

spankalee

Adding a <template> tag later is great for perf too because the contents are still inert and fast to clone.

kitsunesoba

Front end web is not my forte, but as I understand display:none; can have unintended (usually negative) interactions with screen readers, which I don't believe is a possibility with HTML templates.

lelandfe

This is not true. I suspect it is a myth, I've heard it for decades now.

WebAIM[0]:

> display:none or visibility: hidden

> The content is removed from the visual flow of the page and is ignored by screen readers

[0] https://webaim.org/techniques/css/invisiblecontent/#techniqu...

ataylor32

Are you sure? It's my understanding that screen readers know to ignore elements with display:none;

grimgrin

There’s a way to hide from screen readers too if the OP was so inclined:

https://developer.mozilla.org/en-US/docs/Web/Accessibility/A...

cantSpellSober

display: none hides the content from screen readers, as it is not rendered.

aria-hidden is for elements that were rendered but are hidden (it's redundant on elements with a display value of `none`)

cantSpellSober

<template> (along with <slot>) allows for a declarative shadow DOM instead of clunky attachShadow()s on hidden <div>s (which doesn't work server-side)

Otherwise you probably don't need <template>, in fact they don't handle events the same as the rest of the DOM and will likely cause more pain

imaginamundo

I read an article about it that i liked a lot.

https://web.dev/declarative-shadow-dom/

I even tried to build a library around this, but still don’t know how to finish it hahaha.

https://github.com/imaginamundo/webcomponents

rokhayakebe

I assume content within "display:none" will be read by search engines and viewed in source. Is <template> read by search engines?

jamesfinlayson

Search engines would know to ignore the initial <template> but I'm not sure how display: none is handled.

lenkite

I wish HTML and not just JS had supported for consuming templates. That is I wish we could do variants of:

    1. Define Template using <template id="card-template">...</template>
    2. Consume Template using <use-template id="card1" refid="card-template">..</use-template>
    
With substitution variables/text, this feature could be amazing.

blowski

> With substitution variables/text, this feature could be amazing.

At which point, are you just re-creating JavaScript?

dspillett

Not quite, I think he is taking about a more declarative approach compared to that offered by vanilla JS.

vbezhenar

But works with disabled JS.

akira2501

Here's what I got:

    const $template = function(template) {
        // make copy of template content
        const root = template.content.cloneNode(true);
        
        // create proxy object for accessing named nodes and root
        const obj = {
                get $root() {
                        // after template root is called, remove this getter function
                        delete obj.$root;
                        
                        // return root only once
                        return(root);
                }
        };
        
        // find all named template nodes and add to proxy object
        for (const node of root.querySelectorAll('[data-tmpl-name]'))
                obj[node.dataset.tmplName] = node;
                
        // otherwise create template node content wrapped in proxy which
        // makes attempts to overwrite properties an error.
        return(new Proxy(obj, {
                set: () => { throw (new Error(`Attempt to overwrite a template node!`)); }
        }));
    }
    
You can use it with something like:

    <template id="some_id">
        <div>
            <h3 data-tmpl-name="header"></h3>
            <h5 data-tmpl-name="title"></h5>
        </div>
        <p data-tmpl-name="info"></p>
    </template>
And then just:

    function of_some_sort() {
        const t = $template(document.getElementById('some_id'));
        
        t.header.innerHTML = 'The Header';
        t.title.innerHTML = 'The title';
        
        t.info.innerHTML = 'More stuff.';
        
        document.body.append(t.$root);
    }
    
This is was the first version I made. It's not hard to get from here to a version that can fill the template with a data object for you. In my most recent version, you could do:

    function of_another_sort() {
        document.body.append($template(document.getElementById('some_id'), {
            header: 'The Header',
            title:  ['first title', 'second title'],
            
            info: (elem) => {
                elem.setAttribute('functions', 'work');
                elem.innerHTML = 'and receive the internal element itself.';
            }
        }));
    }
    
Arrays duplicate the internal element and make copies of it, objects recurse into the element building a 'key.path.name' while looking for data-tmpl-name to replace. Functions get a copy of the element for more than just innerText/innerHTML replacement. A null or false value eliminates the internal element, and a true value passes it through unchanged.

If you've ever used the old ruby library Amrita, it's basically that, but for HTML Template elements. You can easily do all this in about 100 lines of JS.

vidaj

I just tested out how to use template with slots, and it was as you said quite simple.

Made a simple gist: https://gist.github.com/vidaj/b27f1140a8ae711c7df2372c4b5cfe... It supports using templates as slot content and updating only the slots you want. Add in some event listeners, and you got basic binding support.

pretzelhands

<template> tags are so incredibly useful for keeping your JS clean when you're not able to have something more reactive around for some reason.

I've recently used that on an interview project I did (https://github.com/pretzelhands/ubiquitous-sniffle) and it surprisingly takes you quite far with very little effort.

The only major annoyance is really manually keeping track of the elements in the DOM and .innerText and .innerHTML-ing everything that needs a dynamic value. But it's manageable if you keep it confined.

wildrhythms

This approach always seems so easy at first glance, but I find it gets rather unwieldy as soon as I need to update some deeply nested value- like updating a innerText in cell in a table in a card in a layout of a thousand cards without re-rendering the entire collection. I started using lit-html and it solves this problem (and only costs me 3KB or so to ship).

akrymski

I've used this approach in my minimal MVC lib for over a decade, simply using hidden elements instead of templates

http://krymski.com/espresso.js/

ohmahjong

Is the demo site still supposed to be up? I see

> TypeError: path must be absolute or specify root to res.sendFile

when I try to visit it.

Gualdrapo

I'm redoing from scratch my portfolio using plain JavaScript and <template>s for most stuff, pulling everything from a JSON as a pseudo database. I think for this kind of uses <template> is great, but you can really wish it has some extra features like the ability to define repeating block sections so you didn't need to declare nested <template>s for what would be a single block code.

geuis

Can't this be done more easily / programmatically via createDocumentFragment?

https://developer.mozilla.org/en-US/docs/Web/API/Document/cr...

colejohnson66

That's what the `<template>` element does. From the HTML standard[0]:

> The template element can have template contents, but such template contents are not children of the template element itself. Instead, they are stored in a DocumentFragment associated with a different Document — without a browsing context — so as to avoid the template contents interfering with the main Document.

[0]: https://html.spec.whatwg.org/multipage/syntax.html#template-...

icedchai

In the jquery days, I would often do this sort of thing with hidden divs.

clairity

i remember doing stuff like that too, but now it's official! ha.

it's exciting to see web standards (finally) taking direction from web developers rather than corporate interests, despite chrome being so prominent. as a random aside, i'm especially hoping forms get more love, like the common behaviors (datetime entry, combobox, validation/feedback, etc.) that every developer has to wrangle with over and over. and it's great to see things like the <modal> element becoming almost fully usable without js (it still needs to be triggered via js, but can be closed with a method='dialog' form button).

runarberg

Datetime is mostly there. I think firefox and safari have yet to implement <input type=month> and <input type=week>. As for combobox, <datalist id=mylist> and <input list=mylist> is supported everywhere. However I would like to see the CSSWG focus on better standards for styling these, it is possible now using a lot of vendor-specific hacks, but is really annoying.

I’m gonna go ahead though and declare frontend validation (as good as) finished. the constraint validation API is amazing to work with (if you are not afraid of intercepting the submit event using JavaScript).

clairity

yah, for datetime, i was mostly thinking about styling the widget via css (and perhaps a bit of configuration via json), with the flexibility that all those datetime components had from the jquery days. month and week are indeed just text fields in firefox (just tested it).

and for validation, i'd like it to be js-less, as it's such an integral part of the form use case. there are recent features that help, like :user-invalid and :focus-within, but it's still far from ideal for default behavior. something as simple as styling labels based on validation state of the input is only possible with the advent of :has(), which is still incomplete/experimental in firefox, but even that's still clunky (there's a combinatorial explosion of possible states to cover, having to consider :disabled, :hover, :required, :focus, etc.).

recursive

now? finally?

I've been using <template> for almost a decade.

cantSpellSober

You weren't doing the same thing, though it may have had the same effect. Template elements aren't part of the "main" DOM.

icedchai

It had the same effect, yes, which it what I meant. The hidden div was effectively used as a template, by cloning the element and doing some awful manual data binding from JS.

twism

Man ... I would have been thrown off a cliff if I did this in the 2000s

p2detar

I appreciate this type of posts very much. Short, neat and to the point. I learned something today. Till now, I've always used:

  <script id="app-item-template" type="text/x-custom-template">
I'll definitely have <template> in mind from now on.

Thank you!

asadlionpk

I wrote apps this way before React came along. I hated inserting/updating values by hand with no binding.