Mile
Zero

React's big idea was always the render function. Even at its initial presentation in 2013, the developers were very clear that the original class syntax was just a thing they added to meet contemporary expectations. They also, at the time, stressed that React could be mixed into other code. You could migrate your application over to it piecemeal, taking advantage of the speed improvements they promised in hot spots.

Over the following decade, React took over the whole application space, but conceptually it never moved past render(), and in fact almost everything else was gradually stripped away. When the deprecation of class components removed local state and lifecycle methods, they were replaced with stores like Redux, or contexts, and eventually hooks — all of which are complex and come with a laundry lists of caveats and limitations, but they "solve" the problems caused by eliminating everything that isn't a pure function. The history of the entire project has been constant, downward pressure, moving everything into the view callback. It's all one undifferentiated slab of JSX now.

Perhaps this marks me as a radical, but my thesis is that it may not be beneficial to try to reduce your solutions until they can fit in a cramped, ideologically-constrained display layer. I think it's a good thing when an application has a little flexibility depending on the problems relevant to each part, just as it's good to build a house out of different materials instead of just pouring concrete into a giant mold and calling it a day.

When critics say that web components are incomplete compared to React and its competitors, they're not wrong: if you want One Weird Trick for your entire codebase, you'll be disappointed. But if you're using web components, it may be useful to ask whether you can get many of the benefits of frameworks — live updates, cross-cutting data, loose coupling — without going down the same rabbit holes or requiring the extensive build infrastructure they depend on. It's worth thinking about what we could do if we used the platform to fill those gaps, and for me that starts with events.

Subscribable stores

A common problem: I want to share some state across components that are not located close to each other in the UI tree, and be notified when that state changes so I can re-render.

The most basic solution to this is an event emitter with getter/setter methods wrapping its value. Back in the bad old days, you'd have to roll your own, but EventTarget (the common interface for all DOM classes that dispatch events) has been widely subclassable for a few years now. Our store definition probably looks something like this:

class Store extends EventTarget {
  state = undefined;
  
  constructor(initial) {
    super();
    this.state = initial;
  }

  get value() {
    return this.state;
  }
  
  set value(state) {
    if (this.state == state) return;
    this.state = state;
    this.notify("update", state);
  }
  
  //convenience method for atomic get/set
  update(fn) {
    this.value = fn(this.state)
  }
  
  notify(type, detail) {
    this.dispatchEvent(new CustomEvent(type, { detail }));
  }
}

When we want to use this, it's largely similar to the way that a "context" works in other frameworks. You set up a store in a module, and then in places where that data is important, you import it and either subscribe, update its value, or both. Depending on your base class and your templating, you can even auto-subscribe to it in the course of rendering — remember, addEventListener() automatically de-duplicates listeners, so it's safe to call it redundantly as long as you're passing in the same reference (i.e., use a bound method or a handler object, not a fresh arrow function).

This particular store would need to be adapted if your data is deeply-nested, or if you're planning to mutate it in place, since it only notifies subscribers if the reference identity of its data changes. One option would be to build a proxy-based reactive object, similar to what Vue uses, which can be done in about a hundred lines of code for the basics. You could just import @vue/reactivity, of course, but it's educational to do it yourself.

The subscribable store can be designed with a particular shape of object or collection in mind, and offer methods for working with that shape. In my podcast client, I use a Table class that provides promised-based IndexedDB access and fires events whenever feeds are added, removed, or updated in the database.

My other favorite use case for subscriptions is anything based on external stimuli, such as network polling or push notifications, especially if that external source has rich, non-uniform behavior (say, a socket that syncs state with the server, but also lets you know when your app doesn't have network connectivity so that the UI can disable some features).

This design for reactivity is no longer fashionable, but the pace of JavaScript's pop culture makes it easy to forget that it was only 2019 when Svelte v3 (to pick an example) moved from an explicitly event-driven stores to the current syntax sugar. Behind the scenes, the store contract is still basically an event dispatcher with only one event, it's just hidden by the compiler. If we don't use a compiler, we may have to subscribe by hand, but on the other hand we won't be caught on an update treadmill when the framework devs discover observables (sorry, "runes") four years later.

Personally, I don't think it actually matters very much how you get notified for low-level re-renders — if you wanted to argue that a modern framework uses signals for granular reactivity, that's fine by me — but what I like about standardizing on EventTarget for news about high-level concerns is that it's already familiar, it's free with the browser, and it encourages us to think about changes to data more coherently than "a single value" or "a slice of a big state object."

Broadcast messages

The preoccupation with reducing everything to data transformation is a common blind spot in current front-end frameworks. Data is important, of course — I'm a firm believer in the Linus Torvalds maxim that good programmers worry more about structure than they do code — but sometimes something happens that doesn't create a notable change, or it creates different kinds of changes in different places, or it's a long-running process that we just want to keep an eye on. Not everything is a noun! We need verbs, too!

When I worked on Caret from 2014 to 2018 or so, I was learning a lot about how to structure a relatively large, complex application — certainly it was the biggest thing I'd ever built on my own. But one of the best decisions I made early on was to have different parts of the editor communicate with each other using command messages sent over a central pub/sub (publish and subscribe) channel.

This had a lot of advantages. Major systems didn't need to be tightly coupled together, especially around menus and keyboard shortcuts, which effectively transformed streams of input events into higher-level commands. Some web apps may be able to pretend that the real work is safely isolated from side effects and statefulness, but a programmer's text editor has to deeply care about making input both easy and extensible. And as Caret went from a basic Notepad.exe replacement to a much more full-featured editor, its vocabulary of commands expanded naturally.

Take live settings, for example: Caret saved user preferences in JSON "files," which were persisted to Chrome's synchronized storage. When these changed, the settings provider would send an "init:restart" announcement over the command bus, and modules that used these files would reload their configuration to match. Importantly, the provider did not need to know which systems were listening, or what specific options they cared about (if any). The command was explicit, auditable, and self-explanatory, as opposed to a reactive framework where the settings object changes and a half-dozen other modules spontaneously reload themselves.

Like our subscribable store, the message channel is a subclass of EventTarget. It doesn't retain a value, but it can have a method to simplify the event creation and dispatch process. Instantiate that class, export it from the module, and import it anywhere you want to listen to messages.

class MessageBus extends EventTarget {
  broadcast(type, detail) {
    var e = new CustomEvent(type, { detail });
    this.dispatchEvent(e);
    return e;
  }
}

export const channel = new MessageBus();

I recommend namespacing (and probably defining constants for) your type strings early, since they're going to be sent far and wide: "change" or "update" isn't very useful when lots of things could be changing/updating, but "session:saved" (with the filename attached to the event detail) means you're less likely to collide with other messages, especially on a team.

The main thing I regret from Caret was not having a way for event consumers to send values back to the broadcaster directly. There was an optional callback in the event emitter code, but it was awkward to use, especially if multiple listeners wanted to return values. If I were building it now, I would imitate the Service Worker API and offer a respondWith() method on events:

class RespondableEvent extends Event {
  #responses = [];
  
  constructor(type, data) {
    super(type);
    this.data = data;
  }
  
  respondWith(response) {
    this.#responses.push(response);
  }
  
  // make responses add-only and async
  get responses() {
    return Promise.all(this.#responses);
  }
}

Listeners that need additional time to prepare can respond with a Promise instead of a direct value, meaning that this also doubles as a waitUntil() method. On the other end, the broadcasting module holds onto a reference to the event, and checks to see if it needs to take further action. In Caret, this would have been really useful for providing extension points like language servers or build automation:

var e = new RespondableEvent("file:beforesave", fileContents);
channel.dispatchEvent(e);
// check to see if any plugins responded
var annotations = await e.responses;
// add annotations to the editor control
for (var annotation of annotations) {
  /* ... */
}

Scenarios where asynchronous event responses are necessary are rare, but when you need them, they're invaluable, and this design doesn't add any overhead when not used.

Software as Metaphor

Conway's Law in software development says that the systems designed by an organization are a reflection of its communication structures. I would take that further: the systems we design are, at least a little, a reflection of the way we want the world to work. Part of the reason I like using event-driven architectures is because they effectively create chatty little communities within the program — colonial organisms, like a Portuguese man o' war (though hopefully less dangerous) — and for all my misanthropic tendencies, I do still believe we live in a society.

More importantly, this is a way to think about high-level architecture, but it does not have to be the single method for every part of the app. As I said at the start, I'm suspicious of all-encompassing framework paradigms. If our software is a microcosm of our ideal environment, there's something worrying about reducing all processes to a "pure" transform of input and output.

Web components are not a complete framework in the way that React (or Vue, or Svelte) is. They're just one layer of an application. You can see that as a flaw, but I think it's an opportunity to go back to software that has texture to it, where the patterns that are used at the top level do not have to be the exact same as those used in individual modules, or at lower layers of the stack, if it turns out that they're not well-suited to the problem at hand.

And to be fair, outside of React we see a lot more experimentation with forms of coordination that aren't tied so tightly to one particular VDOM. Preact's signals, for example, provide a level of reactivity that can be used anywhere, and which you could easily integrate with the architecture I've described (listeners updating signal values, and effect functions dispatching events).

I don't think web components are the only reason for that, but I do think their existence as a valid alternative — a kind of perpetual competition to framework code, in which you can get started without a single import statement or npm install — means that there's greater incentive to build primitives that are interoperable, not locked to a single ecosystem.

In the context of that reset, events make sense to me in terms of organizing my code, but I'm hopeful that we'll soon find other techniques re-emerging from the vast prior art of UI toolkits, both native and on the web. There's a lot more out there than closures and currying, and I can't wait to see it.

I have one draft written about framework-free architecture, but it gets a little heavy, and I don't want to go there yet. So instead, here are some minor uses of custom elements that, for me, spark joy.

Set dynamic CSS variables

One of the things that cracks me up when I've been reading about React contexts is how they're almost always demonstrated on (and encouraged for) visual theming. Did nobody tell the developers that CSS... cascades? It's in the name! This is what custom properties are for! (Actual answer: they know and they don't care, because React was designed by people who would rather build countless levels of ideological abstraction than actually use the browser in front of them.)

CSS custom properties have been fairly well-supported since 2016, which is when Chrome and Safari shipped them. When I first started using them, they felt like a step back from the Less variables that I was used to, with the clunky var() syntax required to unwrap them, and calc() to operate on their contents. But custom properties are actually a lot more powerful because they do cascade — you can override them for sections of the DOM tree selectively — and because they can be updated on the fly, either using media queries or values from JavaScript.

Web components are a great way to take dynamic information and make it available for styling, since any CSS properties they set on themselves will then cascade down through the rest of the tree. Imagine we have the following markup:

<mouse-colorizer>
  <h1 style="color: var(--mouse-color, salmon)">
    This space intentionally left blank.
  </h1>
</mouse-colorizer>

If we don't do anything, the inner <h1> will be salmon-colored. But if we define the mouse colorizer element...

class MouseColorizer extends HTMLElement {
  constructor() {
    super();
    this.addEventListener("mousemove", this);
  }

  handleEvent(e) {
    var normalX = e.offsetX / this.offsetWidth;
    var normalY = e.offsetY / this.offsetHeight;
    this.style.setProperty(
      "--mouse-color",
      `rgb(${255 * normalX}, 255, ${255 * normalY})`
    );
  }
}

customElements.define("mouse-colorizer", MouseColorizer);

Now moving the mouse around inside the <mouse-colorizer>'s bounding box will set the color of the headline, between green, cyan, yellow, and white at each corner. Other elements inside this branch of DOM can also use this variable, for any style where a color value is valid: borders, background, shadows, whatever. There are lots of serious uses for being able to set dynamic cascade values, but I think it's just as valuable when it's a little mischievious:

• Parallax effects that shift with the device orientation (GitHub's 404 page used to do this, but they seem to have dropped it lately)
• Animations that progress as the page scrolls
• Tinting images based on the time of day
• Fonts that get just a little blurrier every time you touch the screen

While at NPR, I was working on a project for Louder than a Riot (RIP), one of the company's music podcasts. We wanted to give the page some life, and tie the visuals to the heavy use of audio samples. Animating the whole page with JavaScript was possible, but CSS custom properties gave us an easier solution. I hooked up the player to a WebAudio analyzer node, and had it dispatch events with the average amplitude of the sample window. Then, I set up a <speaker-boxxx> element that listened for those events, and set its --volume property to match. Styles inside of a <speaker-boxxx> could use that value to set color mixes and transforms (or any other style), so that when a track was playing, UI elements all over the page grooved along with it.

Wrap old libraries

A couple of weeks ago I needed a map for a story (people love maps, and although they're rarely the appropriate choice for data visualization, in this case it made sense). Actually, I needed two maps: the reporter wanted to compare the availability of 8th grade algebra over the last ten years.

My go-to library for mapping is Leaflet. It's long in the tooth at this point, and I'm sure that there are other libraries that would offer features like vector tiles or GPU-accelerated rendering. But Leaflet is fast, and it's well-proven, so I've stuck with it.

The thing is, if you need multiple maps, Leaflet can be kind of a pain. It works on the jQuery UI model, where you have to point it at an element and call a factory function to set it up, and then you have a map object that's separate from the DOM that it's attached to. It sure would be nice if the page just took care of that — I don't know, like a callback when the right element gets connected or something.

Well.

class MapElement extends HTMLElement {

  connectedCallback() {
    this.map = new leaflet.Map(this, {
      zoomSnap: .1,
      scrollWheelZoom: false,
      zoomControl: false,
      attributionControl: false
    });
    this.map.focus = () => null;

    leaflet.tileLayer("https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png", {
      subdomains: "abcd".split("")
    }).addTo(this.map);
  }
}

Now I can get a list of maps just by running a query for leaflet-map elements, and populate them based on a filter value that I take from their data-year attributes. It's not that you can't write this in a DRY way, but it feels cleaner to me if I can just hand off the setup to the browser.

We talk a lot about using web components as glue between modern frameworks, but they're also really useful for wrapping up non-framework code that you don't want to think about. If you're maintaining a legacy site that uses older widget libraries, it's worth thinking about whether some of them can be replaced with custom elements to clean up some of the boilerplate and lifecycle management you're currently managing manually.

Add harmless interaction effects

When custom elements were first introduced, Google tried to make them more palatable by packaging them as Polymer, a library of tags and tools. It's hard to say why Google kills anything, but it didn't help that the framework leaned heavily on HTML imports, which (sadly) did not make it out of standardization.

I was pretty lukewarm on Polymer, but one thing I did like was that it had a set of paper-x tags that implemented bits of Material Design, like the ripple effect for clicks that you can still see in some Google UI. These are good candidates for custom elements, because they're client-only and if JavaScript doesn't load, they just don't do anything, but the page still works. They're an easy way to add personality to a design system.

Let's set up our own ripple layer tag as a demonstration. We'll start by creating some shadow DOM and injecting a canvas that's positioned absolutely within its container. We'll also register for pointer events, and bind the "tick" method that runs our animations:

var template = `
<style>
  :host {
    position: relative;
    display: block;
  }

  canvas {
    position: absolute;
    inset: 0;
    width: 100%;
    height: 100%;
    background: transparent;
  }
</style>
<canvas></canvas>
<slot></slot>
`;

class RippleLayer extends HTMLElement {
  ripple = null;

  constructor() {
    super();
    var root = this.attachShadow({ mode: "open" });
    root.innerHTML = template;
    var canvas = root.querySelector("canvas");
    this.context = canvas.getContext("2d");
    this.tick = this.tick.bind(this);
    this.addEventListener("pointerdown", this);
  }
}

With the infrastructure in place, we'll watch for pointer events and start a ripple when one occurs, storing the important information about when and where the click or tap occurred:

handleEvent(e) {
  this.releasePointerCapture(e);
  this.ripple = {
    started: Date.now(),
    x: e.offsetX,
    y: e.offsetY
  }
  this.tick();
}

Finally, we'll add the tick() method that actually draws the ripple. Our animation basically just looks at the current time and draws a circle based on where it should be at that point — we don't need to retain any information from one frame to the next.

tick() {
  // resize the canvas buffer 1:1 with its CSS size
  var { canvas } = this.context;
  canvas.width = this.offsetWidth;
  canvas.height = this.offsetHeight;
  // find out how far in the ripple we've gotten
  var elapsed = Date.now() - this.ripple.started;
  var duration = this.getAttribute("duration") || 300;
  var delta = elapsed / duration;
  // if the ripple is complete, don't draw and stop animating
  if (delta >= 1) return;
  // determine the size of the ripple
  var eased = .5 - Math.cos(delta * Math.PI) / 2;
  var rMax = canvas.width > canvas.height ? canvas.width * .8 : canvas.height * .8;
  // draw a darker outline and lighter inner circle
  this.context.arc(this.ripple.x, this.ripple.y, rMax * eased, 0, Math.PI * 2);
  this.context.globalAlpha = (1 - delta) * .5;
  this.context.stroke();
  this.context.globalAlpha = (1 - delta) * .2;
  this.context.fill();
  // schedule the next update
  requestAnimationFrame(this.tick);
}

Once this element is defined, you can put a <ripple-layer> anywhere you want the effect to occur, such as absolutely positioned in a button. This does require the "layer" to be on top — if you need to have multiple clickable items within a ripple zone, invert the tag by adding a slot, so that the layer wraps around content and adds an effect to it instead of vice-versa.

In my mind, Michael Crichton's Jurassic Park marks the last time object-oriented programming was cool. Dennis Nedry, the titular park's sole computer engineer, adds a backdoor to the system disguised as "a block of code that could be moved around and used, the way you might move a chair in a room." Running whte_rabt.obj as a shell script turns off the security systems and electric fences, kicking off the major crisis that drives the novel forward. Per usual for Crichton, this is not strictly accurate, but it is entertaining.

(Crichton produced reactionary hack work — see: Rising Sun, Disclosure, and State of Fear — roughly as often as he did classic high-tech potboilers, but my favorite petty grudge is in The Lost World, the cash-grab sequel to Jurassic Park, which takes a clear potshot at the "This is a UNIX system, I know this!" scene from Spielberg's film: under siege by dinosaurs, a young woman frantically tries to reboot the security system before suddenly realizing that the 3D graphics onscreen would require a high-bandwidth connection, implying — for some reason — a person-sized maintenance tunnel she can use as an escape route. I love that they can clone dinosaurs, but Jurassic Park engineers do not seem to have heard of electrical conduits.)

In the current front-end culture, class-based objects are not cool. React is (ostensibly) functional wherever possible, and Svelte and Vue treat the module as the primary organizational boundary. In contrast, web components are very much built on the browser platform, and browsers are object-oriented programs. You just can't write vanilla JavaScript without using new, and I've always wondered if this, as much as anything else, is the reason a lot of framework authors seem to view custom elements with such disdain.

Last week, I wrote about slots and shadow DOM as a way to build abstract domain-specific languages and expressive web components. In this post, I want to talk about how base classes and inheritance can smooth out its rough edges, and help organize and arrange the shape of your application. Call me a dinosaur (ha!), but I think they're pretty neat.

Dino DNA

Criticisms of custom elements often center around the amount of code that it takes to write something fairly simple: comparing the 20-line boilerplate of a completely fresh web component against, say, a function with some JSX in it. For some reasons, these comparisons never discuss how that JSX is transpiled and consumed by thousands of lines of framework dependencies — that's just taken for granted — or that some equivalent could also exist for custom elements.

That equivalent is your base class. Rather than inheriting directly from HTMLElement, you inherit from a middleware class that extends it, and fills in the gaps that the browser doesn't directly provide. Almost every project I work on either starts with a base element, or eventually acquires one. Typically, you'll want to include:

• Some kind of templating for the shadow DOM, and optionally for the light DOM.
• Code that reflects observed attributes to properties, or vice versa.
• Method binding, for event listeners and callbacks.
• Event dispatching, using either CustomEvent or a subclass for your application.

If you don't feel capable of providing these things, or you're worried about the maintenance burden, you can always use someone else's. Web component libraries like Lit or Stencil basically provide a starter class for you to extend, already packed with things like reactive state and templating. Especially if you're working on a really big project, that might make sense.

But writing your own base class is educational at the very least, and often easier than you might think, especially if you're not working at big corporate scale. In most of my projects, it's about 50 lines (which I often copy verbatim from the last project), and you can see an example in my guidebook. The templating is the largest part, and the part where just importing a library makes the most sense, especially if you're doing any kind of iteration. That said, if you're mostly manipulating individual, discrete elements, a pattern I particularly like is:

class TemplatedElement extends HTMLElement {
  elements = {};

  constructor() {
    super();
    // get the shadow root
    // in other methods, we can use this.shadowRoot
    var root = this.attachShadow({ mode: "open" });
    // get the template from a static class property
    var { template } = new.target;
    if (template) {
      root.innerHTML = template;
      // store references to marked template elements
      for (var element of root.querySelectorAll("[as]")) {
        var name = element.getAttribute("as");
        this.#elements[name] = element;
      }
    }
  }
}

From here, a class extending TemplatedElement can set a string as the static template property, which will then be used to set up the shadow DOM on instantiation. Any tag in that template with an "as" attribute will be stored on the elements lookup object, where we can then add event listeners or change its content:

class CounterElement extends TemplatedElement {
  static template = `
<div as="counter">0</div>
<button as="increment">Click me!</button>
  `;
  
  #count = 0;
  
  constructor() {
    // run the base class constructor
    super();
    // get our cached shadow elements
    var { increment, counter } = this.elements;
    increment.addEventListener("click", () => {
      counter.innerHTML = this.#count++;
    });
  }
}

It's simple, but it works pretty well, especially for the kinds of less-intrusive use cases that we're seeing in the new wave of HTML components.

For the other base class responsibilities, a good tip is to try to follow the same API patterns that are used in the platform, and more specifically in JavaScript in general (a valuable reference here is the Web Platform Design Principles). For example, when providing method binding and property reflection, I will often build the interface for these as arrays assigned to static properties, because that's the pattern already being used for observedAttributes:

class CustomElement extends BaseClass {
  static observedAttributes = ["src", "controls"];
  static boundMethods = ["handleClick", "handleUpdate"];
  static reflectedAttributes = ["src"];
}

I suspect that once decorators are standardized, they'll be a more pleasant way to handle some of this boilerplate, especially since a lot of the web component frameworks are already doing so via Typescript. But if you're using custom elements, there's a reasonable chance that you're interested in no-build (or minimal build) systems, and thus may want to avoid features that currently require a transpiler.

Clever Girl

But if you're designing larger applications, then your components will need to interact with each other. And in that case, a class is not just a way of isolating some DOM code, it's also a contract between application modules for how they manage state and communication. This is not new or novel — it's the foundation of model-view-controller UI dating back to Smalltalk — but if you've learned web development in the era since Backbone fell out of popularity, you may have never really had to think about state and interaction between components, as opposed to UI functions that all access slices of a common state store (or worse, call out to hooks and magically get served state from the aether).

Here's an example of what I mean: the base class for drawing instructions in Tarot, Chalkbeat's social media image generator, does the normal templating/binding dance in its constructor. It also has some utility methods that most canvas operations will need, such as converting between normalized coordinates and pixels or turning variable-length CSS padding strings into a four-item array. Finally, it defines a number of "stub" methods that subclasses are expected to override:

• persist() and restore() transfer values between elements with the same ID when the user switches card layouts, triggered by the connected and disconnected callbacks.
• getLayout() returns a DOMRect with the bounding box that the component plans to render to, so that parent elements can perform layout tasks like flex spacing.
• draw() actually renders to a canvas context, usually based on the information that getLayout() provided.

When Tarot needs to re-render the canvas, it starts at the top level of the input form, loops through each direct child, and calls draw(). Some instructions, like images or rectangle fills, render immediately and exit. The layout brushes, <vertical-spacer> and <vertical-stack>, first call getLayout() on each of their children, and use those measurements to apply a transform to the canvas context before they ask each child to draw. Putting these methods onto the base class in Tarot makes the process of adding a new drawing type clear and explicit, in a way that (for me) the "grab bag of props" interface in React does not.

Two brushes actually take this a little further. The <series-logo> and <logo-brush> elements don't inherit directly from the Brush base class, but from a specialized subclass of it with properties and methods for storing and tinting bitmaps. As a result, they can take a single-color input PNG and alter its pixels to match any of the theme colors selected while preserving alpha, which means we can add new brand colors to the app and not have to generate all new logo art.

Planning the class as an API contract means that when they're slotted or placed, we can use duck-typing in our higher-level code to determine whether elements should participate in a given operation, by checking whether they have a method name that matches our condition. We can also use instanceof to check if they have the required base class in their prototype chain, which is more strict.

Hold Onto Your Butts

It's worth noting that this approach has its detractors, and has for a (relatively) long time. In 2015, the React team published a blog post claiming that traditional object-oriented code inherently creates tight coupling, and the code required grows "as the square of the number of possible states of the component." Personally I find this disingenuous, especially when you step back and think about the scale of the infrastructure that goes into the "easier" rendering method it describes. With a few small changes, it'd be indistinguishable from the posts that have been written discounting custom elements themselves, so I guess at least they're consistent.

As someone who cut their teeth working in ActionScript 3, it has never been obvious to me that stateful objects are a bad foundation for creating rich interfaces, especially when we look at the long history of animation libraries for React — eventually, every pure functional GUI seems to acquire a bunch of pesky escape hatches in order to do anything useful. Weird how that happens! My hot take is that humans are messy, and so code that interacts directly with humans tends to also be a little messy, and trying to shove it into an abstract conceptual model is likely to fail in frustrating ways. Objects are often untidy, but they give us more slack, and they're easier to map to a mental model of DOM and state relationships.

That said, you can certainly create bad class code, as the jokes about AbstractFactoryFactoryAdapter show. I don't claim to be an expert on designing inheritance — I've never even drawn a UML diagram (one person in the audience chuckles, glances around, immediately quiets). But there are a few basic guidelines that I've found useful so far.

Remember that state is inspectable. If you select a tag in the dev tools and then type $0.something in the console, you can examine a JS value on that element. You can also use console.dir($0) to browse through the entire thing, although this list tends to be overwhelming. In Chrome, the dev tools can even examine private fields. This is great for debugging: I personally love being able to see the values in my application via its UI tree, instead of needing to set breakpoints or log statements in pure rendering functions.

Class instances are great places for related platform objects. When you're building custom elements, a big part of the appeal is that they give you automatic lifecycle hooks for the section of the page tree that they wrap. So this might be obvious, but use your class to cache references to things like Mutation Observers or drawing contexts that are related to the DOM subtree, even if they aren't technically its state, and use the lifecycle to set them up and tear them down.

Use classes to store local state, not application state. In a future post, I want to write about how to create vanilla code that can fill the roles of stores, hooks, and other framework utilities. The general idea, however, is that you shouldn't be using web components for your top-level application architecture. You probably don't need <application-container> or <database-connection>. That's why you...

Don't just write classes for your elements. In my podcast client, a lot of the UI is driven by shared state that I keep in IndexedDB, which is notoriously frustrating to use. Rather than try to access this through a custom element, there's a Table class that wraps the database and provides subscription and manipulation/iteration methods. The components in the page use instances of Table to get access to shared storage, and receive notification events when something else has updated it: for example, when the user adds a feed from the application menu, the listing component sees that the database has changed and re-renders to add that podcast to the list.

Be careful with property/method masking. This is far more relevant when working with other people than if you're writing software for yourself, but remember that properties or methods that you create in your class definitions will supplant any existing fields that exist on HTMLElement For example, on one project, I stored the default slot for a component on this.slot, not realizing that Element.slot already exists. Since no code on the page was checking that property, it didn't cause any problems. But if you're working with other people or libraries that expect to see the standard DOM value, you may not be so lucky.

Consider Symbols over private properties to avoid masking. One way to keep from accidentally overwriting a built-in field name is by using private properties, which are prefixed with a hash. However, these have some downsides: you can't see them in the inspector in Firefox, and you can't access them from subclasses or through Proxies (I've written a deeper dive on that here). If you want to store something on an element safely, it may be better to use a Symbol instead, and export that with your base class so that subclasses can access it.

export const CANVAS = Symbol("#canvas");
export const CONTEXT = Symbol("#context");

export class BitmapElement extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: "open" });
    this[CANVAS] = document.createElement("canvas");
    this[CONTEXT] = this[CANVAS].getContext("2d");
  }
}

The syntax itself looks a little clunkier, but it offers encapsulation closer to the protected keyword in other languages (where subclasses can access the properties but external code can't), and I personally think it's a nice middle ground between actual private properties and "private by convention" naming practices like this._privateButNotReally.

Inherit broadly, not deeply. Here, once again, it's instructive to look at the browser itself: although there are some elements that have extremely lengthy prototype chains (such as the SVG elements, for historical reasons), most HTML classes inherit from a relatively shallow list. For most applications, you can probably get away with just one "framework" class that everything inherits from, sometimes with a second derived class for families of specific functionality (such as embedded DSLs).

There's a part of me that feels like jumping into a wave of interest in web components with a tribute to classical inheritance has real "how do you do, fellow kids?" energy. I get that this isn't the sexiest thing you can write about an API, and it's very JavaScript-heavy for people who are excited about the HTML component trend.

But it also seems clear to me, reading the last few years of commentary, that a lot of front-end folks just aren't familiar with this paradigm — possibly because frameworks (and React in particular) have worked so hard to isolate them from the browser itself. If you try to turn web components into React, you're going to have a bad time. Embrace the platform, learn its design patterns on their own terms, and while it still won't make object orientation cool, you'll find it's a much more pleasant (and stable) environment than it's been made out to be.

Over the last few weeks, there's been a remarkable shift in the way that the front-end community talks about web components. Led by a number of old-school bloggers, this conversation has centered around so-called "HTML components," which primarily use custom elements as a markup hook to progressively enhance existing light DOM (e.g., creating tabs, tooltips, or sortable tables). Zach Leatherman's taxonomy includes links to most of the influential blog posts where the discussions are taking place.

(Side note: it's so nice to see blogging start to happen again! Although it's uncomfortable as we all try to figure out what our new position in the social media landscape is, I can't help but feel optimistic about these developments.)

Overall, this new infusion of interest is a definite improvement from the previous state of affairs, which was mostly framework developers insisting that anything less than a 1:1 recreation of React or Svelte in the web platform was a failure. But the whiplash from "this API is useless because it doesn't bundle enough complexity" to "this API can be used in the simplest possible way" leaves a huge middle ground unexplored, including its most intriguing possibilities.

So in the interest of keeping the blog train rolling, I've been thinking about writing some posts about how I build more complex web components, including single-page apps that are traditionally framework territory, while still aiming for technical accessibility. Let's start by talking about slots, composition, and structure.

Starting from slots

I wrote a little about shadow DOM in 2021, right before NPR published the Science of Joy, which used shadow DOM pretty extensively. Since that time, I've rewritten my podcast client and RSS reader, thrown together an offline media player, developed (for no apparent reason) a Eurorack-esque synthesizer, and written a social card image generator just in time for Twitter to fall apart. Between them, plus the web component book I wrote while wrapping up at NPR, I've had a chance to explore the shadow DOM in much more detail.

I largely stand by what I said in 2021: shadow DOM is a little confusing, not quite as bad as people make it out to be, and best used in moderation. Page content wants to be in the light DOM as much as possible, so that it's easier to style, inspect, and access for scripting. Shadow DOM is analagous to private properties or Symbol keys in JS: it's where you put stuff that only that element (and its user) needs to access but the wider page doesn't know about. But with the addition of slots, shadow DOM is also the way that we can define the relationships of an element to its contents in a way that follows the grain of HTML itself.

To see why, let's imagine a component with what seems like a pointless shadow DOM:

class EmptyElement extends HTMLElement {
  constructor() {
    super();
    var root = this.attachShadow({ mode: "open" });
    root.innerHTML = "<slot></slot>";
  }
}

One (minor) benefit is that it lets you provide automatic fallback content for your element, which is hard to do in the light DOM (think about a list that shows a "no items" message when there's nothing in it). But the more relevant reason is because it gives us access to the slotchange event, as well as methods to get the assigned elements for each slot. slotchange is basically connectedCallback, but for direct children instead the custom element itself: you get notified whenever the elements in a slot are added or removed.

Simple slotting is a great pattern if you are building wrapper elements to enhance existing HTML (similar to the "HTML components" approach noted above). For example, in my offline media player app, the visualizer that creates a Joy Division-like graph from the audio is just a component that wraps an audio tag, like so:

<audio-visuals>
  <audio src="file.mp3"></audio>
</audio-visuals>

When it sees an audio element slotted into its shadow DOM, it hooks it into the analyzer node, and there you go: instant WinAmp visualizer panel. I could, of course, query for the audio child element in connectedCallback, but then my component is no longer reactive, and I've created a tight coupling between the custom element and its expected contents that may not age well (say, a clickable HTML component that expects a link tag, but gets a button for semantic reasons instead).

Configuration through composition

Child elements that influence or change the operation of their parent is a pattern that we see regularly in built-ins:

• Media elements (audio, video, and picture) get live input configuration from <source>
• Subtitles are also loaded by placing a <track> inside an audio or video tag
• Selectbox options on mobile are native UI generated from child elements
• SVG filters contain a list of operation elements, some of which have their own child config tags (think <fePointLight> for the lighting effects, or the <feFuncX> elements in a component transfer)

Tarot, Chalkbeat's social card generator, takes this approach a little further. I talk about this a little in the team blog post, but essentially each card design is defined as an HTML template file containing a series of custom elements, each of which represents a preset drawing instruction (text labels, colored rectangles, images, logos, that kind of thing). For example, a very simple template might be something like:

<vertical-spacer padding="20 0">

  <series-logo color="accent" x=".7" scale=".4"></series-logo>

  <vertical-stack dx="40" anchor="top" x=".4">

    <text-brush
      size="60"
      width=".5"
      padding="0 0 20"
      value="Insert quote text here."
      >Quotation</text-brush>

    <image-brush
      recolor="accent"
      src="./assets/Chalkline-teal-dark.png"
      align="left"
    ></image-brush>
    
  </vertical-stack>

  <logo-brush x=".70" color="text" align="top"></logo-brush>

</vertical-spacer>

<photo-brush width=".4"></photo-brush>

Each of the "brush" elements has its customization UI in its shadow DOM, plus a slot that lets its children show through. The app puts the template HTML into a form so the user can tweak it, and then it asks each of the top-level elements to render. Some of them, like the photo brush, are leaf nodes: they draw their image to the canvas and exit. But the wrapper elements, like the spacer and stack brushes, alter the drawing context and then ask each of their slotted elements to render with the updated configuration for the desired layout.

The result is a nice little domain-specific language for drawing to a canvas in a particular way. It's easy to write new layouts, or tweak the ones we already have. My editor already knows how to highlight the template, because it's just HTML. I can adjust coordinate values or brush nesting in the dev tools, and the app will automatically re-render. You could do this without slots and shadow DOM, but it would be a lot messier. Instead, the separation is clean: user-facing UI (i.e., private configuration state) is in shadow, drawing instructions are in the light.

Patchwork languages

I really started to see the wider potential of custom element DSLs when I was working on my synthesizer, which represents the WebAudio signal path using the DOM. Child elements feed their audio signal into their parents, on up the tree until they reach an output node. So the following code creates a muted sine wave, piping the oscillator tone through a low-pass filter:

<audio-out>
  <fx-filter type="lowpass">
    <source-osc frequency=440></source-osc>
  </fx-filter>
</audio-out>

The whole point of a rack synthesizer is that you can rearrange it by running patch cords between various inputs and outputs. By using slots, these components effectively work the same way: if you drag the oscillator out of the filter in the inspector, the old and new parents are notified via slotchange and they update the audio graph accordingly so that the sine wave no longer runs through the lowpass. The dev tools are basically the patchbay for the synth, which was a cool way to give it a UI without actually writing any visual code.

Okay, you say, but in a Eurorack synthesizer, signals aren't just used for audible sound: the same outputs can be used as control voltage, say to trigger an envelope or sweep a frequency. WebAudio basically replicates this with parameter inputs that accept the same connections as regular audio nodes. All I needed to do to expose this to the document was provide named slots in components:

<fx-filter frequency=200>
  <fx-gain gain=50 slot=frequency>
    <source-osc frequency=1></source-osc>
  </fx-gain>
  <source-osc frequency=440></source-osc>
</fx-filter>

Here we have a similar setup as before, where a 440Hz tone is fed into a filter, but there's an additional input: the <fx-gain> is feeding a control signal with a range of -50 to 50 into the filter's frequency parameter once per second. The building blocks are the same no matter where we're routing a signal, and the code for handling parameter inputs ends up being surprisingly concise since it's able to lean on the primitives that slots provide for us.

The Mask of the Demon

In photography and cinema, the term "chiaroscuro" refers to the interplay and contrast between light and dark — Mario Bava's Black Sunday is one of my favorite examples, with its inky black hallways and innovative color masking effects. I think of the shadow DOM the same way: it's not a replacement for the light DOM, but a complement that can be used to give it structure.

As someone who loves to inject metaphor into code, this kind of thing is really satisfying. By combining slots, shadow DOM, and markup patterns, we can embed a language in HTML that produces either abstract data structures, user interface, or both. Without adding any browser plugins, we're able to manipulate this tree just using the dev tools, so we can easily experiment with our application, and it's compatible with our existing editor tooling too.

Part of the advantage of custom elements is that they have a lower usage floor: they do really well at replacing the kinds of widgets that jQueryUI and Bootstrap used to provide, which don't by themselves justify a full single-page app architecture. This makes them more accessible to the kinds of people that React has spent years alienating with JS-first solutions — and by that, I mean designers, or people who primarily use the kinds of HTML/CSS skills that have been gendered as feminine and categorized as "lesser" parts of the web stack.

So I understand why, for that audience, the current focus is on custom elements that primarily use the light DOM: after all, I started using custom elements in 2014, and it took six more years before I was comfortable with adding shadow DOM. But it's worth digging a little deeper. Shadow DOM and slots are some of my favorite parts of the web component API now, because of the way that they open up HTML as not just a presentational toolkit, but also as an abstraction for expressing myself and structuring my code in a language that's accessible to a much broader range of people.

When it comes to web development, I'm actually fairly traditional. By virtue of the kinds of apps I make (either bespoke visualizations for work or single-serving toys for personal use), I'm largely isolated from a lot of the pain of modern front-end web development. I don't use React, I don't need to scale servers, and I render my HTML the old-fashioned way, from string templates. Even so, my projects are usually built on top of a few build tools, including Rollup, Less, and various SDKs for moving data between different cloud providers.

However, for internal utilities and personal projects over the last few years, I've been experimenting with removing tools, and relying solely on the modern browser. So instead of bundling JS, I'm just loading modules with import statements. I write one CSS file for my light DOM, but custom properties have largely eliminated what I need a preprocessor to do (and the upcoming support for nesting will cover the rest). Add something like the Eleventy dev server for live reload, and it's actually a really pleasant experience.

It's one thing to go minimalist for a single-serving hobby app, or for people in the Chalkbeat newsroom who can reach me directly for support. It's another to do it for a general audience, where the developer/user ratio starts to tilt and your scale becomes more amibitious. But could we develop a real, public-facing web app that doesn't rely on a brittle and slow compilation step? Is a no-build deployment feasible?

While I'm optimistic, I have enough self-awareness to know that things are rarely as simple as I want them to be. I wasn't always a precious snowflake, and I've seen first-hand that national (or international) scale applications have support infrastructure for a reason. To that end, here's a non-exhaustive list of potential hurdles I believe developers will need to jump to get to that tooling-free future.

Caching and coherency

In theory, HTTP2 (which reuses connections and parallelizes transfers) means that we don't pay a penalty for deploying our JavaScript as individual modules instead of a single bundled file. But it raises a new issue that we didn't have with those big bundles: what happens when we make a breaking change in part of the application, and someone visits it with a partially-primed cache, so they have some old files still hanging around? How do we make sure that we can take advantage of caching appropriately, while still keeping our code coherent for a given deployment?

Imagine we have a page that loads module A, which loads B and C, and is styled using CSS file D. I update file B, and changes to D are required for the new components. Different files may be evicted from the browser cache in unpredictable ways, though. Ideally, A and C should be loaded from the cache, and B and D should be fresh requests. If everything comes from the cache, users won't see new features, but ideally nothing should be immediately broken. It would be wasteful, but not disastrous, if all files are loaded fresh. The real problem comes if only one of B or D comes from the cache, so that we either get new code without the matching style changes, or styles without the new code.

As Jake Archibald notes, there are two working (and compatible) strategies for caching interrelated code: either long cache times with unique URLs, or no-cache headers and a shorter lifetime. I lean toward the latter strategy for now, probably using ETag hash-based headers for each file. Individual requests would be a little slower, since the browser would always check the server for individual files, but you'd only actually transfer new code, which is the expensive part (cache hits would return 304 Not Modified). Based on my experience with a similar system for election data updates, I think this would probably scale pretty well, but you'd need to test to be sure.

Once import maps are supported in all evergreen browsers, the hashed URL solution becomes the simpler of the two. Use short identifiers for all your import statements (say, based from the project root), and then hash their contents and generate a JSON mapping between the original path and the mangled filename for production deployments. Now the initial page load can be revalidated on every load, but the scripts that go with that particular page version will be immutable, guaranteeing that any change means a new URL and no cache conflicts. Here's hoping Safari ships import maps to users soon.

Vendor code

Personally, the whole point of developing things in a no-build environment is that I don't need to learn, manage, and optimize around third-party libraries. The web platform is far from perfect, but it's fast and accessible, and there's an undeniable pleasure in writing every line of code. I'm lucky that I have that opportunity.

Most teams are not lucky, and need to load libraries written by other people. Package managers mean we have a wealth of code at our fingertips. But at the same time, the import patterns that work well for Node (lots of modules in a big, deep folder hierarchy) have proven a clumsy match for the front-end. Importing files from node_modules is clumsy and painful, especially if you're also loading stylesheets and other non-JavaScript assets. In fact, much of the tooling explosion (including innovations like tree-shaking and transpilation) comes from trying to have our cake from npm and eat it too.

So loading from the same package manager as the server-side code is frustrating, and using a CDN requires us to trust a remote host completely (plus introducing another DNS/TCP handshake) into our performance waterfall. The ideal would be a shallow set of third-party modules that are colocated with our front-end code, similar to how Bower (RIP) used to handle libraries. Sadly, there are few tools or code conventions that I'm aware of now specifically for that niche anymore.

One approach that I'm intrigued by is Deno's bundle command, which generates an importable module file from an URL, including all its dependencies. Using a tool like this, you could pretty easily zip up vendor code into a single file in the equivalent of src/bower_components. You'd also have a lot more visibility into just how big those third-party libraries are when they're packed up into self-contained (absolute) units, which might provoke a little reflection. Maybe you don't need 3MB of time zone data after all.

That said, one secret weapon for managing those chunky libraries is asynchronous import(). Whereas code-splitting in a bundler is a complicated and niche process, when we use ES modules natively our code is effectively pre-split, and the browser gives us a mechanism to only request libraries when we need them. This means the cost equation for vendor code can change somewhat: maybe it's not great that a given component is multiple megabytes of script, but if users only pay the cost for that transfer and compilation when they're actually going to use it, that's a substantial improvement over the current state of affairs.

CSS imports

I've worked on some large projects where we had a single, unprocessed CSS file for the product. It was hard to stay disciplined. Without nesting or external constraints, we'd end up duplicating styles in different parts of the document and worrying about breakages if we needed to change something. The team tried to keep things well-structured, but you know how it is: if you've got six programmers, you have 12 different ideas about how the site should be organized.

CSS has @import for natively splitting styles into multiple files, but historically it hasn't been considered good for performance. Imports block the renderer and parser, meaning that you may be halting page load for the header while you wait for footer styles. We still want multiple small files on HTTP2, so the best practice is still to generate lots of <link> tags for CSS, possibly using tricks to unblock the parser. Luckily, at least, CSS is not load-order dependent the way that JavaScript is, and the @layer rule gives us ways to manage the cascade. But manually appending a tag for every stylesheet doesn't feel very ergonomic.

I don't have a good solution here. It's possible this is not as serious a problem as I think it is — certainly on my own projects, I'm able to move localized styles into shadow DOM and load them as a part of the component registration, so it tends to solve itself for anything that's heavily interactive or component-based. But I wish @import had the kind of ergonomics and care that its JavaScript counterpart did, and I suspect teams will find PostCSS easier to use than the no-build alternative.

HTML partials and templating

What's the ultimate point of eschewing build tools? Sure, on some level it's to avoid ever touching webpack.config.js (a.k.a. the Lament Configuration) ever again. But it's also about trying to claw our way back from a front-end culture that has neglected the majority of users. And the best way to address an audience on typical devices (read: an Android phone with meager single-core performance and a spotty network connection, or a desktop PC from 2016) is to send less JavaScript and more HTML and CSS.

Last week, I loaded a page from a local news outlet for work, which included data on a subset of Chicago schools. There was no dynamic content, although it did have an autocomplete search at the top. I noticed the browser tab was stuttering on load, so I looked in the dev tools: each of the 600+ schools was being individually templated and appended to a queried element from a JSON fetch. On a fairly new desktop PC, it froze the UI thread for more than half a second. On a phone, that was more like 7 seconds, even with ads blocked, and any news dev will tell you that the absolute easiest way to boost your story's load performance is to remove the ads from it.

If that page had been built as static HTML, it would parse and load almost instantly by comparison. Indeed, in the newsroom projects that I maintain, the most important feature is the ability to pull in data from a variety of sources (Google Docs, Sheets, local text and CSV, JSON, remote APIs) and merge that easily with the HTML template. The build scripts do other things, like bundling and CSS processing and deployment. But those things could be replaced, or reduced, or moved into other tools without radically changing the experience. HTML generation is irreplaceable.

At a bare minimum, let's say I want to be able to include partial templates (for sharing headers and snippets between pages), loop through some data, and inject my import map or my stylesheet collection into the page. Here's a list of the tools that let me do that easily, on most Linux servers, without installing a bunch of extra crap:

• PHP

Listen, no shade on PHP, but I don't want to write it for a living anymore even if I wasn't working off of static file storage. It's a hard sell, especially in the context of "a modern web stack."

HTML templating is where the rubber really meets the road. We do not have capabilities for meta-processing in the language itself, and any solution that involves JavaScript (including the late, lamented HTML imports) is a non-starter. I'd kill for an <include> tag, especially if there were a way to use it without blocking the parser, similar to the way that declarative shadow DOM provides declarative support for component subtrees.

What I'm not interested in doing is stripping the build toolchain down if it means a worse experience for users. And once I need some kind of infrastructure to assemble my HTML, it's not actually that much more work to bolt on a script bundler and a stylesheet preprocessor, and reap the benefits from those ecosystems. I'm all-in on the web platform, but I'm not a masochist.

Let's build

This is by no means an exhaustive list of challenges, but Nano tells me I'm well past 200 lines in this text document, so let's wrap it up.

The good news, as I see it, is that the browser is in a healthier place than ever for hobbyists, students, and small project developers. You can open index.html, import Lit or Vue from a CDN, and have a reasonably performant front-end environment that can be grow more complex to fit your needs and skills. You can also write a lot less JavaScript than in years past, because CSS has gotten so much better for layout and interaction.

I'd say we're within reach of a significantly less complicated front-end technical culture. I would not be surprised to see companies start to experiment with serving JavaScript or CSS directly, using tooling to smooth off the rough edges (e.g., producing import maps or automating stylesheet inclusion) rather than leaning hard into full, slow-moving compilation steps. The ergonomics of these approaches are going to be better than a lot of people expect. Some front-end teams that have specialized in tooling-intensive ecosystems are going to either eat a lot of crow or get very angry for a while.

All that said: we're not going back to the days when all you needed was notepad.exe and some moxy to make a "real" website. Perhaps it's naive to think we ever were. But making a good web app is hard, I would argue harder than many other kinds of programming. It's the code you write in a trio of languages, but also the network between you and the user, the management of distributed state, and a vast range of devices, inputs, and outputs. The least we can do is make it less wearying to get started.

Through a confluence of issues, Safari (Apple's web browser, and the only browser allowed on iOS) has been a hot topic lately:

• Multiple game streaming services have rolled out in the browser instead of through a centralized app store on iPhones, including Microsoft's Xbox Cloud. These are probably the highest profile web-only apps on iOS in years, and ironically Safari only recently became capable of hosting them.
• iOS 14.1.1 shipped with a showstopper bug in the IndexedDB API, part of a long stream of bugs that break Safari's ability to store data locally and work offline. Because browser releases are tied to the OS, developers will have to work around this for at least half a year and probably more (since many users don't upgrade promptly).
• The Safari team asked for feedback about what new features developers would like to prioritize, which reminded everyone that the existing features are largely broken and it's part of a systemic pattern of neglect and abuse. Lord knows I have my own collection of horror stories.

When these kinds of teapot tempests stir up, you can often sort the reaction from the technical community into a few buckets. At the extreme "actually, Safari is good" side, there are people who argue that the web should be replaced or downgraded into something more like Gemini, or restricted to the feature set of HTML 4 and CSS 2 (no scripting allowed). You know: cranks.

But you'll also see a second group proposing that "browsers should be for documents, not for apps" (e.g. browser developers should just stop adding new features entirely and let's split the web in two). In this line of thinking, a browser like Safari that refuses (or is slow) to implement new APIs or features is doing the world a service, because it keeps the ecosystem tilted toward the "document" side instead of the "app" platform side, where Google has too much influence. These opinions seem more reasonable on the surface, but they're also cranks — it's just harder to explain why.

The flaw in the "document browsers, not app platforms" argument is that it assumes that web APIs can be sorted into clear, easily distinguished buckets — or indeed, that there's a bright line between the two. In fact, as someone who almost entirely builds content pages (jargon about "news apps" aside), I often find that in conversations with "app" developers that I'm more experienced with new browser APIs than they are. Most client-side apps, like GMail or Trello, do not actually use that much of a browser's API surface. Even really ambitious applications like Figma mostly just need methods for storage and display, and they've had those (through IndexedDB and canvas) for at least a decade now.

Should browsers be simpler and easier to implement? This kind of argument often feels very intuitive to the "document web" advocates, because they're used to thinking about new APIs through the context of the marketing bullet points for a new operating system. But when you actually look over a list at Can I Use, an awful lot of the "new" APIs are just paving cowpaths: they're designed to replace or reduce common patterns that developers were already hacking onto pages.

• Beacon API - lets you fire a request at a server without waiting for a response, which means that developers can stop intercepting link clicks and pausing navigation while they send an analytics ping.
• Fetch - makes it easier to safely load information from a server, replacing XMLHttpRequest (which was hard to use) and JSONP (which was a security nightmare).
• Intersection Observer - lets developers know when an element has entered or exited the visual viewport without having to poll constantly, which means scrolling gets smoother.
• Web Crypto - keeps people from shipping huge crypto libraries as a part of their JS bundle, and supports privacy-first features like end-to-end encryption.
• Web Assembly - creates a stable compilation target for other languages. Developers were already creating other languages that compile to JavaScript, Web Assembly just creates a standard interface and a predictable performance profile.
• Web Sockets - replaces previous methods of getting fast updates on events, such as constant polling requests or persistent server connections that would take down Apache.
• Various message channels - lets developers communicate between tabs without abusing sidechannels like window.name or local storage, useful for all the people who have GMail open in seven tabs because they never close anything.
• Grid and flex layouts - replaces various hacks and JavaScript-based layout systems, including the holy grail: vertically-centered content.

Because JavaScript is a Turing-complete language and web browsers were originally designed with lots of holes in them, none of these APIs are really adding anything new to the browser — it's just that previously, this functionality would have been added by brute force. For example, before browsers created consistent ways to autoplay video without loading a large and dithered .gif file, there were scripts to "play" frames via canvas and a tiled .jpg. You'd be amazed the hacks like this I've seen (and some I've perpetrated).

Are there APIs in Chrome that cross into traditional native app territory? Sure, there's a few, like the Bluetooth or USB access APIs. But while pundits and native developers seem to think those are the vast majority of new browser features, I think it's clear from the listings (and my own experience) that those don't actually represent very much usage in modern apps (they're only about 1/10 of the items on the Can I Use index of JS APIs). They're certainly not what most people complaining about Safari are actually talking about.

What's especially jarring for me, as a visual journalist, is that the same people who rail against the complexity of the web platform will often praise the interactive stories from teams like mine. While I appreciate the support, I can't help but feel that they think our work is less technically challenging or innovative than a "real" developer's, and that they're happy to have a browser push the envelope only as long as it doesn't pose any competition to Apple's revenue stream.

In contrast, if you look at something like my parents' hometown paper (with an ad-blocker, of course), it's not far off from the "document web" ideal — and it looks unbelievably quaint. Despite the warm glow of nostalgia around "the old web" when men were men, browsers were small, and pages were laid out in tables, actually returning to that standard would feel like trying to use DOS for a day: clumsy, slow, and ugly.

That's why when someone says "browsers should be for pages, not for apps," we should ask specifically what they mean by that:

• Do they mean physically handing Word files around, like we did before Google Docs? Can anyone imagine going back to a native office suite for any kind of collaboration?
• Are slippy maps okay, or should we go back to the Mapquest experience of clicking a little arrow and waiting for the page to reload in order to see a little more to the east?
• Do you want responsive charts in your news articles, so that they're legible on any device? Think of all the COVID explainers and election results from the past year — should all those have been rejected for being "too app-like?"
• Should a person be able to check their e-mail from any computer, or should they have to install a dedicated native client and remember all their server details?
• Think about all the infrequent tasks you do online, and now imagine that they're all either regressed to the 1998 version or built as native code. Do you think you should have to install an app just to book a flight? To buy a book? To find a new job?

(Incidentally, it's wild how much the mobile market has been distorted on these issues: I think most people would consider it a total non-starter to need to install a desktop app to read Facebook or stream a TV show, but Apple has worked very hard to protect their platform from browser-based options on mobile.)

I think it's possible for someone to look at that list and still insist that yes, they want browsers to be Gopher clients with slightly better font choices. I personally doubt it, though — I suspect most people making the case for a "document-first" web aren't irrational, they just like the romance of the idea and haven't fully thought it through. I sympathize! That doesn't mean we have to take them seriously.

There isn't, in my opinion, a cooler name for a web standard than the Shadow DOM. The closest runner-up is probably the SubtleCrypto API, and after a decade of Bitcoin the appeal of anything with "crypto" in the name is pretty cloudy. So it's a low bar, but still: Shadow DOM. Pretty cool name.

Although I've been using web components for a long time, I've only been using Shadow DOM with it for a couple of years, in generally in pretty limited ways. For an upcoming project at NPR, I took the chance to really dig into how it's used in a mixed-content environment, one where custom elements are not just leaves of the HTML tree, but also wrap branches of extensive HTML content. The experience was pretty eye-opening, and surprisingly positive!

Walking the pattern

Let's start by talking about what what it is. Like most of the tech under the web components "brand," Shadow DOM is meant to retroactively give developers tools that "explain" what the browser already does, and hook into the same extension points. The goal is to make it possible for regular people to rapidly build out new functionality, because there's no "magic" behind the scenes.

For example, let's create a humble <select> tag:

Choose your fighter Marx Engels

Right off the bat, this tag has some special treatment that we can't immediately explain through regular HTML: it has a "thumb" (the arrow on the right) that doesn't appear in the DOM and can't be meaningfully styled, but is clearly a UI element that reacts to events. The options, defined as children of the tag, are still surfaced visibly, but not in the same way that children of a paragraph are or a regular text element are. Instead, they're moved to a new location in the dropdown menu and shown conditionally (or, on mobile, through an entirely different UI context).

Using our previous HTML/JS toolkit, it's not possible to duplicate these behaviors, or similar behaviors from tags like <video> or <input type="range">. To explain the "magic" of these elements we need to add Shadow DOM. It gives developers an API to attach a hidden document fragment called a "shadow root" to any given element, which replaces the visible contents of the element. However, even though they're shown to us in the browser, the contents of that document fragment are hidden from normal JavaScript queries, and its CSS styles are isolated — from the inside, you have a blank slate to work from, and from the outside it's as though that shadow content is an intrinsic part of the tag itself, just like the select box's dropdown UI.

What about those select box options, which are written as child tags but appear in a very different way? For that, we add in a <slot> element: inside the shadow, this element will re-parent any children placed in the host element. For example, given a shadow-dom element with the following in its shadow root: <b> SHADOW START </b> <slot></slot> <b> SHADOW END </b>

We could write this in our page as: <shadow-dom> <i>HELLO WORLD</i> </shadow-dom>

The contents of the <i> element aren't shown directly. Instead, they're moved inside the slot element, meaning that the page output will read SHADOW START HELLO WORLD SHADOW END. But, and this is the cool part, that italic tag appears to scripts and dev tools as though it was just a regular child of the <shadow-dom> element — it can be styled as normal, you can query for it, attach event listeners, and edit it as normal. The bold tags, meanwhile, remain in the shadow: they're visible on the page, but they can't be accessed from scripts and their styles are completely isolated.

This, then, is how Shadow DOM "explains" how a select box works. The box itself, including the current item and the thumb UI, live in the shadow. The options you write into the tag are reparented to a slot inside the drop-down area, to be shown when you click the element. We can use this API to create self-contained UI for an application or document without having to worry about new markup or styles polluting the page.

Enter the Logrus

Not everything is rosy, of course. One long-standing complication is that custom elements can't touch their own contents or attributes during construction, for reasons that are tedious and not worth going into here, but they can attach and modify their shadow root. So it's really tempting in custom elements to do everything in a shadow, because it radically simplifies your templating. Now you have null problems. In Radio, I built the entire UI this way, which worked great until I needed to inspect an element that's inside three nested shadow roots, or if I needed to query for the current active element.

Another misunderstanding has been people thinking shadow roots can replace something like Styled Components in terms of style isolation. But Shadow DOM is more like an iframe than anything else: explicitly inherited style properties (like font family) will travel through, but otherwise it's a pretty hard barrier. If you want to provide styling hooks for a component, you need either provide preset options or document a set of CSS custom properties. More importantly, the mechanisms for injecting styles into a shadow root (typically by putting a <style> tag inside) don't play well with standard build tooling.

By contrast, actually populating Shadow DOM tends to be cumbersome without build tooling in place to help. A lot of tutorials recommend building it from an inert <template> tag, which used to be elegantly handled via HTML imports. Now that those are deprecated, you either have to place the Shadow DOM template in your page manually (no), lean into async component definition (awkward), embed the markup into your script as a big JS literal (ugly), or use a build plugin to pull strings in as needed (sigh). None of these are unworkable, or even that difficult, but none of them are nearly as nice as simply being able to define a component's styles, shadow markup, and behavior in a single, imported HTML file.

Major Arcana

My personal feeling is that the biggest barrier to effective Shadow DOM usage, in a lot of cases, is that many developers haven't learned about the browser as much as they've learned about React or another framework, and those frameworks have often diverged in philosophy from the DOM. If you're used to thinking of the page as a JSX function value, the idea of a secret, stateful document fragment that replaces the DOM you tried to render is probably pretty bizarre.

But as someone who writes a lot of minimalist code directly against browser APIs, I actually think Shadow DOM fits in well with my mental model of how elements work, and it has clarified a lot of my thinking on how to build effectively with custom elements — especially through slots and slotted elements.

I'm still learning and experimenting, but I feel comfortable saying that if you're building custom elements, the rule of thumb should be "use Shadow DOM, but not very much." The more you're able to expose HTML to the light DOM by surfacing it through slots, the easier it is to compose them and style content. For example, a custom element that creates a tabbed UI from its children is a great Shadow DOM use case: the tab list lives in the shadow and is generated implicitly by iterating over the slotted elements. Since the actual tab contents are placed back in the light DOM, they're still easy to style and inspect. To really go with the grain of the platform, the host component might show or hide those slotted blocks using the hidden attribute, instead of setting styles or adding classes.

The exception is for elements that should not have children (like input tags) or where children are used for configuration — think video tags or my old Leaflet map component. With these "leaf" components, Shadow DOM lets you treat inner HTML as a domain-specific language, while your visible content lives entirely in the shadow root. That's a great way to create customized behavior, but expose it to designers or novice front-end developers who are very comfortable with markup but would balk at writing a lot of JS.

Ultimately, Shadow DOM feels like it really crystallizes the role of custom elements as a tool for implementing UI widgets, not as a competitor for Svelte. Indeed, by providing a mechanism for moving complex functionality into an opaque facade, it's probably the biggest gift to the "web pages are for documents, not apps" crowd in several years: if you want to build a big single page app, Shadow DOM doesn't really move the needle, but it's great for injecting discrete units of content into an article. As someone who crosses that app/document divide a lot, I'm really excited to see what I can do with it this year.

The past few months, I've mostly been writing in public for NPR's News Apps team blog, with posts on the new Dailygraphics rig (and setting it up on Windows), the Mueller report redactions, and building a scrolling audio story. However, in my personal time, I decided to listen to some podcasts. So naturally, I built a web-based listener app, just for me.

I had a few goals for Radio as I was building it. The first was my own personal use case, in which I wanted to track and listen to a few podcasts, but not actually install a dedicated player on my phone. A web app makes perfect sense for this kind of ephemeral use case, especially since I'm not ever really offline anymore. I also wanted to try building something entirely using Web Components instead of a UI framework, and to use modern features like import — in part because I wanted to see if I could recommend it as a standard workflow for younger developers, and for internal newsroom tools.

Was it a success? Well, I've been using it to listen to Waypoint and Says Who for the last couple of months, so I'd say on that metric it proved itself. And I certainly learned a lot. There are definitely parts of this experience that I can whole-heartedly recommend — importing JavaScript modules instead of using a bundler is an amazing experience, and is the right kind of tradeoff for the health of the open web. It's slower (equivalent to dynamic AMD imports) but fast enough for most applications, and it lets many projects opt entirely out of beginner-unfriendly tooling.

Not everything was as smooth. I'm on record for years as a huge fan of Web Components, particularly custom elements. And for an application of this size, it was a reasonably good experience. I wrote a base class that automated some of the rough edges, like templating and synchronizing element attributes and properties. But for anything bigger or more complex, there are some cases where the platform feels lacking — or even sometimes actively hostile.

For example: in the modern, V1 spec for custom elements, they're not allowed to manipulate their own contents until they've been placed in the page. If you want to skip the extra bookkeeping that would require, you are allowed to create a shadow root in the constructor and put whatever HTML you want inside. It feels very much like this is the workflow you're supposed to use. But shadow DOM is harder to inspect at the moment (browser tools tend to leave it collapsed when inspecting the page), and it causes problems with events (which do not cross the shadow DOM boundary unless you alter your dispatch code).

There's also no equivalent in Web Components for the state management that's core to most modern frameworks. If you want to pass information down to child components from the parent, it either needs to be set through attributes (meaning you only get strings) or properties (more bookkeeping during the render step). I suspect if I were building something larger than this simple list-of-lists, I'd want to add something like Redux to manage data. This would tie my components to that framework, but it would substantially clean up the code.

Ironically, the biggest hassle in the development process was not from a new browser feature, but from a very old one: while it's very easy to create an audio tag and set its source to any sound clip on the web, actually getting the list of audio files is often impossible, thanks to CORS. Most podcasts do not publish their episode feeds with the cross-origin header set, so the browser's security settings shut down the AJAX requests completely. It's wild that in 2019, there's still no good way to make a secure request (say, one that transmits no cookies or custom headers) to another domain. I ended up running the final app on Glitch, which provides basic Node hosting, so that I could deploy a simple proxy for feed data.

For me, the neat thing about this project was how it brought back the feeling of hackability on the web, something I haven't really felt since I first built Caret years ago. It's so easy to get something spun up this way, and that's a huge incentive for creating little personal apps. I love being able to make an ugly little app for myself in only a few hours, instead of needing to evaluate between a bunch of native apps run by people I don't entirely trust. And I really appreciated the ways that Glitch made that easy to do, and emphasized that in its design. It helps that podcasting, so far, is still a platform built on open web tech: XML and MP3. More of this, please!

Last month, I spoke at the first SeattleJS conference, and talked about how we've developed our tools and philosophy for the Seattle Times interactives team. The slides are available here, if you have trouble reading them on-screen.

I'm pretty happy with how this talk turned out. The first two-thirds were largely given, but the last section went through a lot of revision as I wrote and polished. It originally started as a meditation on training and junior developers, which is an important topic in both journalism and tech. Then it became a long rant about news organizations and native applications. But in the end, it came back to something more positive: how the web is important to journalism, and why I believe that both communities have a lot to gain from mutual education and protection.

In addition to my talk, there are a few others from Seattle JS that I really enjoyed:

• Ashley Williams: JavaScript close to the metal - a typically charming look at abstraction, operating systems, and the web platform
• Steve Kinney: Building musical isntruments with Web Audio - A really funny talk on building and playing a modular synthesizer
• Shagufta Gurmukhdas: Web-based VR - Although I'm still not convinced of the value of WebVR, I do love the way this talk shows how custom elements create a beginner-friendly language for new tech

In early August, I delivered my talk on "custom elements in production" to the CascadiaFest crowd. We've been using these new web platform features at the Seattle Times for more than two years now, and I wanted to share the lessons we've learned, and encourage others to give them a shot. Apart from some awkward technical problems with the projector, I actually think the talk went pretty well:

One of the big changes in the web component world, which I touched on briefly, is the transition from the V0 API that originally shipped in Chrome to the V1 spec currently being finalized. For the most part, the changeover is not a difficult one: some callbacks have been renamed, and there's a new function used to register the element definition.

There is, however, one aspect of the new spec that is deeply problematic. In V0, to avoid complicated questions around parser timing and integration, elements were only defined using a prototype object, with the constructor handled internally and inheritance specified in the options hash. V1 relies instead on an ES6 class definition, like so: class CustomElement extends HTMLElement { constructor() { super(); } } customElements.define("custom-element", CustomElement);

When I wrote my presentation, I didn't think that this would be a huge problem. The conventional wisdom on classes in JavaScript is that they're just syntactic sugar for the existing prototype system — it should be possible to write a standard constructor function that's effectively identical, albeit more verbose.

The conventional wisdom, sadly, is wrong, as became clear once I started testing the V1 API currently available behind a flag in Chrome Canary. In fact, ES6 classes are not just a wrapper for prototypes: specifically, the super() call is not a straightforward translation to older inheritance models, especially when used to extend browser built-ins as it does here. No matter what workarounds I tried, Chrome's V1 custom elements implementation threw errors when passed an ES5 constructor with an otherwise valid prototype chain.

In a perfect world, we would just use the new syntax. But at the Seattle Times, we target Internet Explorer 10 and up, which doesn't support the class keyword. That means that we need to be able to write (or transpile to) an ES5 constructor that will work in both environments. Since the specification is written only in terms of classes, I did what you're supposed to do and filed a bug against the spec, asking how to write a backwards-compatible element definition.

It shouldn't surprise me, but the responses from the spec authors were wildly unhelpful. Apple's representative flounced off, insisting that it's not his job to teach people how to use new features. Google's rep closed the bug as irrelevant, stating that supporting older browsers isn't their problem.

Both of these statements are wrong, although only the second is wrong in an interesting way. Obviously, if you work on standards specifications, it is part of your job to educate developers. A spec isn't just for browsers to implement — if it were, it'd be written in a machine-readable language like WebIDL, or as a series of automated tests, not in stilted (but still recognizable) English. Indeed, the same Google representative that closed my issue previously defended the "tutorial-like" introductory sections elsewhere. Personally, I don't think a little consistency is too much to ask.

But it is the dismissal of older browsers, and the spec's responsibility to them, that I find more jarring. Obviously, a spec for a new feature needs to be free to break from the past. But a big part of the Extensible Web Manifesto, which directly references web components and custom elements, is that the platform should be explainable, and driven by feedback from real web developers. Specifically, it states:

Making new features easy to understand and polyfill introduces a virtuous cycle:

• Developers can ramp up more quickly on new APIs, providing quicker feedback to the platform while the APIs are still the most malleable.
• Mistakes in APIs can be corrected quickly by the developers who use them, and library authors who serve them, providing high-fidelity, critical feedback to browser vendors and platform designers.
• Library authors can experiment with new APIs and create more cow-paths for the platform to pave.

In the case of the V1 custom elements spec, feedback from developers is being ignored — I'm not the only person that has complained publicly about the way that the class-based definitions are a pain to use in a mixed-browser environment. But more importantly, the spec is actively hostile to polyfills in a way that the original version was not. Authors currently working to shim the V1 API into browsers have faced three problems:

1. Calling super() invokes magic that's hard to reproduce in ES5, and needlessly so.
2. HTMLElement isn't a callable function in older environments, and has to be awkwardly monkey-patched.
3. Apple publicly opposes extending anything other than the generic HTMLElement, and has only allowed it into the spec so they can kill it later.

The end result is that you can write code that will work in old and new browsers, but it won't exactly look like real V1 code. It's not a true polyfill, more of a mini-framework that looks almost — but not exactly! — like the native API.

I find this frustrating in part for its inelegance, but more so because it fundamentally puts the lie to the principles of the extensible web. You can't claim that you're explaining the capabilities of the platform when your API is polyfill-hostile, since a polyfill is the mechanism by which we seek to explain and extend those capabilities.

More importantly, there is no surer way to slow adoption of a web feature than to artificially restrict its usage, and to refuse to educate developers on how to use it. The spec didn't have to be this way: they could detail ES5 semantics, and help people who are struggling, but they've chosen not to care. As someone who literally stood on a stage in front of hundreds of people and advocated for this feature, that's insulting.

Contrast the bullying attitude of the custom elements spec authors with the advocacy that's been done on behalf of Service Worker. You couldn't swing a dead cat in 2016 without hitting a developer advocate talking up their benefits, creating detailed demos, offering advice to people trying them out, and talking about how they gracefully degrade in older browsers. As a result, chances are good that Service Worker will ship in multiple browsers, and see widespread adoption, by the end of next year.

Meanwhile, custom elements will probably languish in relative obscurity, as they've done for many years now. It's a shame, because I'd argue that the benefits of custom elements are strong enough to justify using them even via the old V0 polyfill. I still think they're a wonderful way to build and declare UI, and we'll keep using them at the Times. But whatever wider success they achieve will be despite the spec, not because of it. It's a disgrace to the idea of an extensible web. And the authors have only themselves to blame.